Add files using upload-large-folder tool

.gitattributes

@@ -454,3 +454,4 @@ janus/lib/python3.10/site-packages/sympy/utilities/tests/__pycache__/test_wester
 janus/lib/python3.10/site-packages/sympy/solvers/tests/__pycache__/test_solveset.cpython-310.pyc filter=lfs diff=lfs merge=lfs -text
 janus/lib/python3.10/site-packages/sympy/solvers/__pycache__/solveset.cpython-310.pyc filter=lfs diff=lfs merge=lfs -text
 janus/lib/python3.10/site-packages/torch/_decomp/__pycache__/decompositions.cpython-310.pyc filter=lfs diff=lfs merge=lfs -text
+janus/lib/python3.10/site-packages/torch/onnx/__pycache__/symbolic_opset9.cpython-310.pyc filter=lfs diff=lfs merge=lfs -text

janus/lib/python3.10/site-packages/torch/distributed/_composable/checkpoint_activation.py

@@ -0,0 +1,126 @@
+# mypy: allow-untyped-decorators
+# mypy: allow-untyped-defs
+from contextlib import contextmanager, nullcontext
+from typing import Any, ContextManager, Dict, Optional, Tuple
+
+import torch
+import torch.nn as nn
+from torch.utils.checkpoint import (
+    _checkpoint_without_reentrant_generator,
+    _DEFAULT_DETERMINISM_MODE,
+)
+
+from .contract import contract
+
+
+@contextmanager
+def _no_hook(module: nn.Module, user_ctx: Optional[ContextManager] = None):
+    r"""
+    Disable hooks installed by checkpoint to avoid unintentional recursion
+    during backward recomputation.
+    """
+
+    with user_ctx if user_ctx else nullcontext():
+        orig_enable_hook = checkpoint.state(module).enable_hook
+        checkpoint.state(module).enable_hook = False
+        try:
+            yield
+        finally:
+            checkpoint.state(module).enable_hook = orig_enable_hook
+
+
+@contract()
+def checkpoint(module: nn.Module, **kwargs) -> nn.Module:
+    r"""
+    This is a composable activation checkpointing API. Unlike functional
+    activation checkpointing APIs, this one does not require changing model
+    source code. Unlike ``nn.Module`` wrapper activation checkpointing APIs,
+    this one does not modify model structure or fully-qualified names either.
+    Under the hood, it registers activation checkpointing logic as pre- and
+    post-forward hooks. Hence, this API can be easily applied to any model or
+    sub-modules in the model.
+
+    Args:
+        module (nn.Module): the target model or sub-module to apply activation
+            checkpointing.
+
+    Example::
+        >>> # xdoctest: +SKIP
+        >>> import torch.nn as nn
+        >>>
+        >>> class MyModel(nn.Module):
+        >>>     def __init__(self) -> None:
+        >>>         super().__init__()
+        >>>         self.l1 = nn.Linear(10, 10)
+        >>>         self.l2 = nn.Linear(10, 10)
+        >>>
+        >>>     def forward(self, x):
+        >>>         return self.l2(self.l1(x))
+        >>>
+        >>> model = MyModel()
+        >>> checkpoint(model.l1)  # apply activation checkpointing only to l1
+        >>> model(torch.zeros(2, 10)).sum().backward()
+
+    """
+    torch._C._log_api_usage_once("torch.distributed.checkpoint")
+
+    use_reentrant = kwargs.pop("use_reentrant", False)
+    if use_reentrant:
+        raise NotImplementedError(
+            "use_reentrant=True is not supported in composable checkpoint. "
+            "Please use torch.utils.checkpoint.checkpoint instead."
+        )
+    preserve_rng_state = kwargs.pop("preserve_rng_state", True)
+    user_context_fns = kwargs.pop("context_fn", None)
+    determinism_check = kwargs.pop("determinism_check", _DEFAULT_DETERMINISM_MODE)
+    debug = kwargs.pop("debug", False)
+
+    if kwargs:
+        raise ValueError(
+            "Unexpected keyword arguments: " + ",".join(arg for arg in kwargs)
+        )
+
+    def forward_pre_hook(
+        module: nn.Module, args: Tuple[Any, ...], kwargs: Dict[str, Any]
+    ) -> None:
+        if checkpoint.state(module).enable_hook:
+
+            def context_fns():
+                if user_context_fns is not None:
+                    ctx1, ctx2 = user_context_fns()
+                    return ctx1, _no_hook(module, ctx2)
+                else:
+                    return nullcontext(), _no_hook(module)
+
+            checkpoint.state(
+                module
+            )._ac_generator = _checkpoint_without_reentrant_generator(
+                module,
+                preserve_rng_state,
+                context_fns,
+                determinism_check,
+                debug,
+                *args,
+                **kwargs,
+            )
+            next(checkpoint.state(module)._ac_generator)
+
+    def forward_hook(module: nn.Module, inputs: Tuple[Any, ...], output: Any) -> Any:
+        if checkpoint.state(module).enable_hook:
+            try:
+                next(checkpoint.state(module)._ac_generator)
+            except StopIteration:
+                pass
+            else:
+                raise RuntimeError(
+                    "Expected non-reentrant activation checkpoint generator to be exhausted, but it was not!"
+                )
+
+        #  Ensure that we no longer hold on to the generator. always_call=True helps ensure we
+        # clear this even in the case of exception in fwd pass.
+        checkpoint.state(module)._ac_generator = None
+
+    checkpoint.state(module).enable_hook = True
+    module.register_forward_pre_hook(forward_pre_hook, with_kwargs=True)
+    module.register_forward_hook(forward_hook, prepend=True, always_call=True)
+    return module

janus/lib/python3.10/site-packages/torch/distributed/_composable/contract.py

@@ -0,0 +1,224 @@
+# mypy: allow-untyped-defs
+import uuid
+from collections import OrderedDict
+from functools import wraps
+from typing import Callable, Dict, List, Optional, Sequence, Type, Union
+
+import torch
+import torch.nn as nn
+from torch.distributed._composable_state import _State
+from torch.distributed.utils import _get_root_modules
+
+
+def generate_state_key(string="__composable_api_state_key"):
+    return f"{string}_{str(uuid.uuid4())}"
+
+
+STATE_KEY = generate_state_key()
+REGISTRY_KEY = generate_state_key()
+
+
+# TODO: we can add additional info to RegistryItem to share across APIs. E.g.,
+# we can add args and kwargs here, and then we can detect whether fully_shard
+# is combined with reentrant activation checkpointing and error out with a clear
+# message.
+class RegistryItem:
+    pass
+
+
+def contract(state_cls: Type[_State] = _State):
+    r"""
+    Decorate a function as a composable distributed API, where the first
+    argument of the function must be an :class:`nn.Module` instance or sequence
+    of :class:`nn.Module` instances.
+
+    The decorator verifies that the decorated function does not modify
+    fully-qualified names (FQNs) for parameters, buffers, or modules. The
+    decorated function can return different module instances than the input
+    modules; the FQN invariant will be enforced following the input order.
+
+    When a function ``func`` is decorated by ``@contract()``, a
+    ``.state(module: nn.Module)`` method will be installed to the decorated
+    function. Then you can retrieve and modify the state on a module by calling
+    ``func.state(module)``.
+
+    Example::
+        >>> # xdoctest: +SKIP
+        >>> import torch.nn as nn
+        >>>
+        >>> class MyModel(nn.Module):
+        >>>     def __init__(self) -> None:
+        >>>         super().__init__()
+        >>>         self.l1 = nn.Linear(10, 10)
+        >>>         self.l2 = nn.Linear(10, 10)
+        >>>
+        >>>     def forward(self, x):
+        >>>         return self.l2(self.l1(x))
+        >>>
+        >>> @contract()
+        >>> def my_feature(module: nn.Module) -> nn.Module:
+        >>>     my_feature.state(module).some_state = "any value"
+        >>>     return module
+        >>>
+        >>> model = MyModel()
+        >>> my_feature(model.l1)
+        >>> assert my_feature.state(model.l1).some_state == "any value"
+        >>> my_feature(model.l2)
+        >>> model(torch.randn(2, 10)).sum().backward()
+    """
+
+    # wraps will make functions decorated with contract() pickleable - needed for integration with torch.package
+    @wraps(state_cls)
+    def inner(func):
+        @wraps(func)
+        def wrapper(
+            module: Union[nn.Module, Sequence[nn.Module]], *args, **kwargs
+        ) -> Optional[nn.Module]:
+            inp_module = module
+            if isinstance(module, nn.Module):
+                modules = [module]
+            else:
+                # If the user passes a sequence of modules, then we assume that
+                # we only need to insert the state object on the root modules
+                # (i.e. those without a parent) among the passed-in modules.
+                modules = _get_root_modules(list(module))
+            state = state_cls()  # shared across all modules
+            registry_item = RegistryItem()  # shared across all modules
+
+            # `func` is allowed to return different module instances than the
+            # input modules as long as FQNs are preserved following the input
+            # module order
+            all_orig_named_params: List[Dict[str, nn.Parameter]] = []
+            all_orig_named_buffers: List[Dict[str, torch.Tensor]] = []
+            all_orig_named_modules: List[Dict[str, nn.Module]] = []
+
+            for module in modules:
+                default_all_state: Dict[Callable, _State] = OrderedDict()
+                default_registry: Dict[str, RegistryItem] = OrderedDict()
+                all_state: Dict[Callable, _State] = module.__dict__.setdefault(  # type: ignore[call-overload]
+                    STATE_KEY, default_all_state
+                )
+                if not isinstance(all_state, dict):
+                    raise AssertionError(
+                        f"Distributed composable API states corrupted: {all_state}"
+                    )
+                registry: Dict[str, RegistryItem] = module.__dict__.setdefault(  # type: ignore[call-overload]
+                    REGISTRY_KEY, default_registry
+                )
+                if not isinstance(registry, dict):
+                    raise AssertionError(
+                        f"Distributed composable API registry corrupted: {registry}"
+                    )
+                if func in all_state or func.__name__ in registry:
+                    raise AssertionError(
+                        "Each distinct composable distributed API can only be applied to a "
+                        f"module once. {func.__name__} has already been applied to the "
+                        f"following module:\n{module}"
+                    )
+                all_state.setdefault(func, state)
+                registry.setdefault(func.__name__, registry_item)
+
+                all_orig_named_params.append(OrderedDict(module.named_parameters()))
+                all_orig_named_buffers.append(OrderedDict(module.named_buffers()))
+                all_orig_named_modules.append(OrderedDict(module.named_modules()))
+
+            updated = func(inp_module, *args, **kwargs)
+            if updated is None:
+                updated = inp_module
+            if isinstance(updated, nn.Module):
+                updated_modules = [updated]
+            else:
+                updated_modules = _get_root_modules(list(inp_module))
+
+            all_new_named_params: List[Dict[str, nn.Parameter]] = []
+            all_new_named_buffers: List[Dict[str, torch.Tensor]] = []
+            all_new_named_modules: List[Dict[str, nn.Module]] = []
+            for module in updated_modules:
+                all_new_named_params.append(OrderedDict(module.named_parameters()))
+                all_new_named_buffers.append(OrderedDict(module.named_buffers()))
+                all_new_named_modules.append(OrderedDict(module.named_modules()))
+
+            num_orig_modules = len(all_orig_named_modules)
+            num_new_modules = len(all_new_named_modules)
+            if num_orig_modules != num_new_modules:
+                raise AssertionError(
+                    f"{func.__name__} should return the same number of modules as input modules"
+                    f"Inputs: {num_orig_modules} modules\n"
+                    f"Outputs: {num_new_modules} modules"
+                )
+
+            def check_fqn(orig_fqns: List[str], new_fqns: List[str], check_key: str):
+                if orig_fqns == new_fqns:
+                    return
+
+                orig_fqn_set, new_fqn_set = set(orig_fqns), set(new_fqns)
+                orig_only = orig_fqn_set - new_fqn_set
+                new_only = new_fqn_set - orig_fqn_set
+                if len(orig_only) or len(new_only):
+                    raise RuntimeError(
+                        f"{check_key}"
+                        "Composable distributed API implementations cannot modify FQNs.\n"
+                        f"FQNs only in original: {orig_only}\n"
+                        f"FQNs only in new: {new_only}"
+                    )
+                else:
+                    raise RuntimeError(
+                        f"{check_key}"
+                        "Composable distributed API implementations cannot modify "
+                        "the order of FQNs.\n"
+                        f"Original FQNs: {orig_only}\n"
+                        f"New FQNs: {new_only}"
+                    )
+
+            for orig_named_params, new_named_params in zip(
+                all_orig_named_params, all_new_named_params
+            ):
+                check_fqn(
+                    list(orig_named_params.keys()),
+                    list(new_named_params.keys()),
+                    "Checking parameters: ",
+                )
+            for orig_named_buffers, new_named_buffers in zip(
+                all_orig_named_buffers, all_new_named_buffers
+            ):
+                check_fqn(
+                    list(orig_named_buffers.keys()),
+                    list(new_named_buffers.keys()),
+                    "Checking buffers: ",
+                )
+            for orig_named_modules, new_named_modules in zip(
+                all_orig_named_modules, all_new_named_modules
+            ):
+                check_fqn(
+                    list(orig_named_modules.keys()),
+                    list(new_named_modules.keys()),
+                    "Checking modules: ",
+                )
+
+            # TODO: verify that installed distributed paradigms are compatible with
+            # each other.
+
+            return updated
+
+        def get_state(module: nn.Module) -> Optional[_State]:
+            return module.__dict__.setdefault(  # type: ignore[call-overload]
+                STATE_KEY,
+                {},  # TODO(@yhcharles): this is a temporary fix, need a better way
+            ).get(
+                func
+            )  # type: ignore[call-overload]
+
+        wrapper.state = get_state  # type: ignore[attr-defined]
+
+        return wrapper
+
+    return inner
+
+
+def _get_registry(module: nn.Module) -> Optional[Dict[str, RegistryItem]]:
+    r"""
+    Get an ``OrderedDict`` of composable APIs that have been applied to the
+    ``module``, indexed by the API name. If no API has been applied, then this
+    returns ``None``.
+    """
+    return getattr(module, REGISTRY_KEY, None)

janus/lib/python3.10/site-packages/torch/distributed/_composable/fsdp/__init__.py

@@ -0,0 +1,2 @@
+from ._fsdp_api import CPUOffloadPolicy, MixedPrecisionPolicy, OffloadPolicy
+from .fully_shard import FSDPModule, fully_shard, register_fsdp_forward_method

janus/lib/python3.10/site-packages/torch/distributed/_composable/fsdp/_fsdp_common.py

@@ -0,0 +1,152 @@
+# mypy: allow-untyped-defs
+import math
+import traceback
+from dataclasses import dataclass
+from enum import auto, Enum
+from typing import Any, cast, List, Optional
+
+import torch
+import torch._dynamo.compiled_autograd as ca
+import torch.distributed as dist
+import torch.nn as nn
+from torch.distributed._composable.contract import _get_registry
+from torch.distributed.tensor import DeviceMesh, DTensor
+from torch.distributed.tensor._dtensor_spec import DTensorSpec
+
+
+@dataclass
+class DataParallelMeshInfo:
+    mesh: DeviceMesh
+    shard_mesh_dim: Optional[int] = None
+    replicate_mesh_dim: Optional[int] = None
+
+    def __post_init__(self):
+        if self.shard_mesh_dim is None and self.replicate_mesh_dim is None:
+            raise AssertionError(
+                "At least one of shard_mesh_dim and replicate_mesh_dim must not be None"
+            )
+
+
+@dataclass
+class FSDPMeshInfo(DataParallelMeshInfo):
+    def __post_init__(self):
+        super().__post_init__()
+        if self.shard_mesh_dim is None:
+            raise AssertionError("Expects non-None shard_mesh_dim")
+        self.shard_mesh_size: int = self.mesh.size(self.shard_mesh_dim)
+        self.shard_process_group = self.mesh.get_group(self.shard_mesh_dim)
+        self.shard_mesh_rank: int = self.shard_process_group.rank()
+
+
+@dataclass
+class DDPMeshInfo(DataParallelMeshInfo):
+    def __post_init__(self):
+        super().__post_init__()
+        if self.replicate_mesh_dim is None:
+            raise AssertionError("Expects non-None replicate_mesh_dim")
+        self.replicate_mesh_size: int = self.mesh.size(self.replicate_mesh_dim)
+        self.replicate_process_group = self.mesh.get_group(self.replicate_mesh_dim)
+        self.replicate_mesh_rank: int = self.replicate_process_group.rank()
+
+
+@dataclass
+class HSDPMeshInfo(FSDPMeshInfo, DDPMeshInfo):
+    def __post_init__(self):
+        # Calls `FSDPMeshInfo` -> `DDPMeshInfo` -> `DataParallelMeshInfo`
+        super().__post_init__()
+
+
+class TrainingState(Enum):
+    """Describes the training state of one FSDP state / parameter group."""
+
+    # Transition to forward starting pre-forward until post-forward
+    FORWARD = auto()
+    # Transition to pre-backward when unsharding in backward
+    PRE_BACKWARD = auto()
+    # Transition to post-backward when resharding and reducing gradients
+    POST_BACKWARD = auto()
+    # Idle before/after forward or before pre-backward/after post-backward
+    IDLE = auto()
+
+
+def _raise_assert_with_print(*args: Any, **kwargs: Any):
+    print(f"[Rank {dist.get_rank()}] ", end="")
+    print(*args, **kwargs)
+    traceback.print_stack()
+    raise AssertionError(*args, **kwargs)
+
+
+def _is_composable_with_fsdp(module: nn.Module) -> bool:
+    registry = _get_registry(module)
+    if registry is None:
+        return True
+    # Registry keys by function name
+    return "replicate" not in registry
+
+
+def _get_dim0_padded_size(tensor_size: torch.Size, dim0_factor: int) -> torch.Size:
+    padded_dim0 = math.ceil(tensor_size[0] / dim0_factor) * dim0_factor
+    return cast(torch.Size, torch.Size([padded_dim0]) + tensor_size[1:])
+
+
+def _chunk_with_empty(
+    tensor: torch.Tensor, num_chunks: int, dim: int
+) -> List[torch.Tensor]:
+    chunks = list(torch.chunk(tensor, num_chunks, dim=dim))
+    while len(chunks) < num_chunks:
+        chunks.append(chunks[0].new_empty(0))
+    return chunks
+
+
+def _get_dim0_chunked_size(
+    chunk: torch.Tensor, unchunked_size: torch.Size
+) -> torch.Size:
+    if chunk.numel() > 0:
+        return chunk.size()
+    # For 0 numel, we need to preserve trailing dims for DTensor APIs
+    return cast(torch.Size, torch.Size([0]) + unchunked_size[1:])
+
+
+def _from_local_no_grad(
+    local_tensor: torch.Tensor,
+    sharding_spec: DTensorSpec,
+) -> DTensor:
+    """
+    This method is similar to ``DTensor.from_local()`` except that in eager mode
+    it avoids some CPU overhead by avoiding default args and not being differentiable.
+    """
+
+    if not ca.compiled_autograd_enabled:
+        return DTensor(
+            # Use the local tensor directly instead of constructing a new tensor
+            # variable, e.g. with `view_as()`, since this is not differentiable
+            local_tensor,
+            sharding_spec,
+            requires_grad=local_tensor.requires_grad,
+        )
+    else:
+        return DTensor.from_local(
+            local_tensor,
+            sharding_spec.mesh,
+            sharding_spec.placements,
+            shape=sharding_spec.shape,
+            stride=sharding_spec.stride,
+        )
+
+
+def _to_dtype_if_needed(
+    tensor: torch.Tensor, dtype: Optional[torch.dtype]
+) -> torch.Tensor:
+    if dtype is not None and tensor.dtype != dtype:
+        return tensor.to(dtype)
+    return tensor
+
+
+def _cast_fp_tensor(dtype: torch.dtype, x: torch.Tensor) -> torch.Tensor:
+    if (
+        not isinstance(x, torch.Tensor)
+        or not torch.is_floating_point(x)
+        or x.dtype == dtype
+    ):
+        return x
+    return x.to(dtype)

janus/lib/python3.10/site-packages/torch/distributed/_composable/fsdp/_fsdp_param.py

@@ -0,0 +1,754 @@
+# mypy: allow-untyped-defs
+import itertools
+from dataclasses import dataclass, field
+from enum import auto, Enum
+from typing import Any, cast, List, Optional, Sequence, Tuple
+
+import torch
+import torch._dynamo.compiled_autograd as ca
+import torch.nn as nn
+from torch._prims_common import make_contiguous_strides_for
+from torch.distributed._functional_collectives import AsyncCollectiveTensor
+from torch.distributed.tensor import DTensor, Replicate, Shard
+from torch.distributed.tensor._dtensor_spec import DTensorSpec, TensorMeta
+from torch.distributed.tensor.device_mesh import _mesh_resources
+from torch.distributed.tensor.placement_types import _StridedShard, Placement
+
+from ._fsdp_api import CPUOffloadPolicy, MixedPrecisionPolicy, OffloadPolicy
+from ._fsdp_common import (
+    _chunk_with_empty,
+    _from_local_no_grad,
+    _get_dim0_chunked_size,
+    _raise_assert_with_print,
+    _to_dtype_if_needed,
+    FSDPMeshInfo,
+    HSDPMeshInfo,
+)
+
+
+"""
+[Note: FSDP tensors]
+FSDP considers the following tensors:
+- Original parameter: parameter passed to :class:`FSDPParam`, i.e. the one
+  on the module when applying FSDP
+- Sharded parameter: sharding the original parameter on dim-0 as a DTensor
+  over the main mesh
+- All-gather inputs: the ``torch.Tensor`` or ``Tensor`` s passed to all-gather,
+  derived from the sharded parameter
+- All-gather output: the ``torch.Tensor`` or ``Tensor`` s resulting from
+  all-gathering the all-gather inputs
+- Unsharded parameter: parameter used for forward/backward computation, derived
+  from the all-gather output; autograd leaf
+
+We define these tensors to describe the general framework that can accomodate
+extensions, where:
+- all-gather-inputs = pre-all-gather-transform(sharded-parameter)
+- unsharded-parameter = post-all-gather-transform(all-gather-outputs)
+
+For the default ``torch.Tensor`` case, there is only one all-gather input, and
+it shares the same underlying tensor data as the sharded parameter, meaning
+that they can be thought of as the same tensors. The same applies for the
+all-gather output and unsharded parameter. For non-``torch.Tensor`` extensions,
+these equivalences may no longer hold due to the pre/post-all-gather
+transforms, and some may have multiple all-gather inputs/outputs (e.g.
+quantized data and scales).
+
+[Note: FSDP and autograd]
+FSDP dynamically frees and allocates the unsharded parameter. Since autograd
+can pack a reference to it or a view to save for backward, we use storage
+resizing to implement the freeing/allocation since that preserves the aliasing.
+This implies that we construct the unsharded parameter object once and write to
+it in-place thereafter. For the default ``torch.Tensor` original parameter
+case, the all-gather output and unsharded parameter share the same
+data, so we use storage resizing on the all-gather output.
+"""
+
+lib = torch.library.Library("fsdp", "FRAGMENT")  # noqa: TOR901
+
+lib.define("set_(Tensor(a!) tensor, Tensor data) -> ()")
+
+
+@torch.library.impl(lib, "set_", "Meta")
+@torch.library.impl(lib, "set_", "CUDA")
+@torch.library.impl(lib, "set_", "CPU")
+def set_(tensor, data):
+    tensor.set_(data)
+
+
+"""
+[Note: Avoiding functionalization for fsdp.set_ and inductor.resize_storage_bytes_(0)]
+
+Currently we don't functionalize `fsdp.set_` op or `inductor.resize_storage_bytes_(0)` op
+(i.e. they show up as a mutation op in the middle of the AOT joint graph).
+
+Reason:
+Traceable FSDP2 compiled autograd BWD graph have the following traits:
+(1) Two inputs of the graph were aliased to each other (one from hook closed-over tensors, one from FWD saved tensors).
+(2) One of them is mutated (set_ and resize_(0) to handle the all-gathered param).
+(3) They are both subclasses.
+The combination of these traits is not supported by AOTAutograd (it's difficult to reason about subclass aliasing).
+So this doesn't work at all for Traceable FSDP2.
+
+The compromise we use is to avoid functionalization for the FSDP2 set_ and resize_(0) ops.
+This avoids the problem above, because from AOTAutograd point-of-view there are no mutations
+that functionalization needs to handle. (Although we need to be careful not to DCE those mutable ops.)
+
+We can avoid this functionalization because:
+(1) The nn.Parameter is never used before its .set_() is called in eager code (i.e. no alias of it is created),
+so it's safe to call .set_() in the middle of the graph to swap out its storage and start using the nn.Parameter downstream.
+(2) We always re-allocate the buffer for nn.Parameter to store the AllGather output and to be used in downstream user ops.
+So calling resize-to-0 in the middle of the graph to free nn.Parameter memory after use should always be okay
+(since we always allocate anew next time we need it, we strictly don't need to keep the old tensor storage around anymore).
+
+Q: But doesn't the torch.compile stack have the "functional graph" assumption in many places?
+A: Yes - this is WIP but we will try to get back to functional graph as early as possible in the lowering process.
+Specifically, we believe we can move both .set_ and .resize_(0) ops to end of graph in AOT joint graph before partitioner
+(i.e. effectively "re-functionalizing" those ops). Put it in another way, we avoid functionalization for those two ops just to
+make AOTAutograd alias analysis happy, and as soon as we are past that point, we "re-functionalize" the graph.
+This requires a custom FX pass but we believe it's not hard to write and maintain.
+
+Q: What's the importance of partitioner not saving views of nn.Parameter as FWD saved tensors?
+A: This is critical: we do want to save FWD nn.Parameter graph input (instead of its view) for BWD use,
+so that downstream ops in BWD graph uses the post-`.set_` nn.Parameter instead of any of its saved views as input.
+This is because .set_ will not update any of the nn.Parameter's views, so BWD downstream ops must use the original
+nn.Parameter in order to see the result of .set_.
+"""
+
+
+@torch.library.impl(lib, "set_", "Functionalize")
+def set__functionalize(tensor, data):
+    torch._sync(tensor)
+    torch._sync(data)
+    # AOTDispatcher needs to know if any inputs had their storages mutated.
+    # (Why? It sometimes detaches inputs before sending them into the graph,
+    #  when it sees that they do not need to have any gradients computed)
+    torch._functionalize_set_storage_changed(tensor)
+    tensor_inner = torch._from_functional_tensor(tensor)
+    data_inner = torch._from_functional_tensor(data)
+    with torch._C._ExcludeDispatchKeyGuard(
+        torch._C.DispatchKeySet(torch._C.DispatchKey.Functionalize)
+    ):
+        torch.ops.fsdp.set_.default(tensor_inner, data_inner)
+
+
+torch.fx.node.has_side_effect(torch.ops.fsdp.set_.default)
+
+
+class ShardedState(Enum):
+    """
+    - ``SHARDED``: The sharded parameter is registered to the module. It is the
+      only contributor to parameter memory.
+    - ``SHARDED_POST_FORWARD``: The unsharded parameter is resharded to a
+      smaller world size. Since this data should not be used for computation,
+      we do not register it to the module. Users should reshard the module
+      before any in-place modifications. Both it and the sharded parameter
+      contribute to parameter memory.
+    - ``UNSHARDED``: The unsharded parameter is registered to the module. Both
+      it and the sharded parameter contribute to parameter memory.
+    """
+
+    SHARDED = auto()
+    SHARDED_POST_FORWARD = auto()
+    UNSHARDED = auto()
+
+
+@dataclass
+class ParamModuleInfo:
+    """
+    For a parameter, this stores the module and the parameter name to be able
+    to do a parameter swap via ``setattr(module, param_name, ...)`` or to get
+    the parameter via ``getattr(module, param_name)``. We additionally save
+    shared modules and shared parameter names to update them accordingly.
+    """
+
+    # Parameter names are unprefixed, e.g. "weight", not "lin.weight"
+    module: nn.Module
+    param_name: str
+    shared_modules: List[nn.Module] = field(default_factory=list)
+    shared_param_names: List[str] = field(default_factory=list)
+
+
+@dataclass
+class ExtensionsData:
+    # User-defined metadata passed from pre to post-all-gather
+    all_gather_metadata: Optional[Any] = None
+    # Save the all-gather input sizes to unflatten the all-gather outputs to ND
+    all_gather_input_sizes: Sequence[torch.Size] = ()  # ND
+
+    def clear(self):
+        self.all_gather_metadata = None
+        self.all_gather_input_sizes = ()
+
+
+class FSDPParam:
+    """
+    This class manages a parameter with FSDP or FSDP variants applied,
+    implementing dim-0 per-parameter sharding.
+    """
+
+    orig_dtype: torch.dtype
+    param_dtype: Optional[torch.dtype]
+    reduce_dtype: Optional[torch.dtype]
+    _orig_size: torch.Size  # ND
+    sharded_size: torch.Size  # ND
+    contiguous_sharded_stride: Tuple[int, ...]
+    padded_sharded_param_size: torch.Size  # ND
+    sharded_post_forward_size: torch.Size  # ND
+    contiguous_sharded_post_forward_stride: Tuple[int, ...]
+    _sharded_param_data: torch.Tensor  # 1D
+    sharded_param: nn.Parameter  # ND
+    _sharded_post_forward_param_data: Optional[torch.Tensor]  # 1D
+    _sharded_post_forward_param: Optional[nn.Parameter]  # ND
+    _unsharded_param: nn.Parameter  # ND
+    unsharded_accumulated_grad: Optional[torch.Tensor]  # ND
+    _sharding_spec: DTensorSpec
+    # DTensor attributes (only defined for DTensor `param`):
+    _tp_spec: DTensorSpec
+    all_gather_outputs: List[torch.Tensor]  # 1D
+    # All-gather extension attributes
+    _extensions_data: ExtensionsData
+    _unsharded_inner_tensors: List[torch.Tensor]
+
+    def __init__(
+        self,
+        param: nn.Parameter,
+        module_info: ParamModuleInfo,
+        mesh_info: FSDPMeshInfo,
+        post_forward_mesh_info: Optional[FSDPMeshInfo],
+        device: torch.device,
+        mp_policy: MixedPrecisionPolicy,
+        offload_policy: OffloadPolicy,
+    ):
+        self._module_info: ParamModuleInfo = module_info
+        self.mesh_info = mesh_info
+        self.post_forward_mesh_info = post_forward_mesh_info
+        self.device = device
+        self.offload_to_cpu: bool = isinstance(offload_policy, CPUOffloadPolicy)
+        self.pin_memory = (
+            self.offload_to_cpu and cast(CPUOffloadPolicy, offload_policy).pin_memory
+        )
+        self.grad_offload_event: Optional[torch.cuda.Event] = None
+        self._init_sharded_param(param, device)
+        if self.post_forward_mesh_info:
+            self._init_sharded_post_forward_param_metadata(param)
+        self._init_extensions()
+        self.all_gather_outputs: List[torch.Tensor] = []
+        self.unsharded_accumulated_grad = None
+        self._param_fqn: Optional[str] = None  # prefixed from root module
+        # TODO: Remove this padding logic once DTensor pads the local tensor:
+        # https://github.com/pytorch/pytorch/issues/113045
+        self._post_load_hook_handle = (
+            module_info.module.register_load_state_dict_post_hook(
+                lambda *args, **kwargs: self.reset_sharded_param()
+            )
+        )
+
+    @torch.no_grad()
+    def _init_sharded_param(self, param: nn.Parameter, device: torch.device):
+        if param.device != device and param.device.type != "meta":
+            raise AssertionError(
+                f"Expects the parameter to already be moved to device {device} but got {param.device}"
+            )
+        # TODO: Replace the sharded DTensor parameter construction logic with
+        # `distribute_tensor` after https://github.com/pytorch/pytorch/issues/116101
+        # TODO: Simplify the following sharded parameter padding logic after
+        # https://github.com/pytorch/pytorch/issues/113045
+        self.is_dtensor = isinstance(param, DTensor)
+        if self.is_dtensor:
+            self._tp_spec = cast(DTensor, param)._spec
+            dp_mesh, tp_mesh = (self.mesh_info.mesh, self._tp_spec.mesh)
+            dp_global_mesh = _mesh_resources.get_root_mesh(dp_mesh)
+            tp_global_mesh = _mesh_resources.get_root_mesh(tp_mesh)
+            if dp_global_mesh != tp_global_mesh or (
+                dp_global_mesh is None or tp_global_mesh is None
+            ):
+                raise AssertionError(
+                    "FSDP requires the DP and TP mesh to have the same parent mesh but got: \n"
+                    f"DP's global mesh: {dp_global_mesh}\nTP's global mesh: {tp_global_mesh}"
+                )
+
+            name_dims_error = "FSDP requires named DeviceMesh dims for ND parallelism"
+            assert dp_mesh.mesh_dim_names is not None, name_dims_error
+            assert tp_mesh.mesh_dim_names is not None, name_dims_error
+            submesh_names = dp_mesh.mesh_dim_names + tp_mesh.mesh_dim_names
+            self._spmd_mesh = dp_global_mesh[submesh_names]
+            if len(self._tp_spec.placements) != 1:
+                raise NotImplementedError(
+                    f"FSDP only supports 1D TP, not {self._tp_spec.placements}"
+                )
+            split_factor = self._tp_spec.num_shards_map[0]
+            assert (
+                2 <= self._spmd_mesh.ndim <= 3
+            ), f"_spmd_mesh.ndim can only be 2 or 3 but got {self._spmd_mesh.ndim}."
+            self._spmd_placements: Tuple[Placement, ...]
+            dp_shard_tp_placement = (
+                (
+                    _StridedShard(0, split_factor=split_factor)
+                    if split_factor > 1
+                    else Shard(0)
+                ),
+                self._tp_spec.placements[0],
+            )
+            if self._spmd_mesh.ndim == 2:
+                self._spmd_placements = dp_shard_tp_placement
+            else:
+                assert self.mesh_info.replicate_mesh_dim == 0
+                self._spmd_placements = (Replicate(),) + dp_shard_tp_placement
+            self._sharding_spec = DTensorSpec(
+                self._spmd_mesh,
+                self._spmd_placements,
+                tensor_meta=self._tp_spec.tensor_meta,
+            )
+            # NOTE: FSDP+TP does not support uneven sharding for now
+            # TODO: enable uneven sharding for FSDP+TP
+            if split_factor > 1:  # FSDP has strided sharding on tensor dim 0
+                num_shards = self._sharding_spec.num_shards_map[0]
+                tensor_size_dim_0 = self._sharding_spec.shape[0]
+                if tensor_size_dim_0 % num_shards != 0:
+                    raise NotImplementedError(
+                        "FSDP+TP sharding does not support uneven sharding for now: "
+                        f"tensor dim 0 has size {tensor_size_dim_0} which cannot be "
+                        f"evenly sharded into {num_shards} shards."
+                    )
+
+            param_data = cast(DTensor, param)._local_tensor
+        else:
+            self._spmd_mesh = self.mesh_info.mesh
+            if isinstance(self.mesh_info, HSDPMeshInfo):
+                self._spmd_placements = (Replicate(), Shard(0))
+            else:
+                self._spmd_placements = (Shard(0),)
+            self._sharding_spec = DTensorSpec(
+                self._spmd_mesh,
+                self._spmd_placements,
+                tensor_meta=TensorMeta(
+                    param.size(),
+                    param.stride(),
+                    param.dtype,
+                ),
+            )
+            param_data = param
+        self._orig_size = param_data.size()
+        self._contiguous_orig_stride = make_contiguous_strides_for(self._orig_size)
+        shard_rank = self.mesh_info.shard_mesh_rank
+        shard_world_size = self.mesh_info.shard_mesh_size
+        chunks = _chunk_with_empty(param_data, shard_world_size, dim=0)
+        sharded_param = chunks[shard_rank]
+        self.sharded_size = _get_dim0_chunked_size(sharded_param, param_data.size())
+        self.contiguous_sharded_stride = make_contiguous_strides_for(self.sharded_size)
+        padded_sharded_size = chunks[0].size()  # 0th always padded
+        padded_sharded_param = param_data.new_zeros(padded_sharded_size)
+        self.padded_sharded_param_size = padded_sharded_param.size()
+        if sharded_param.numel() > 0:
+            padded_sharded_param[: sharded_param.size(0)].copy_(sharded_param)
+        if self.offload_to_cpu and not padded_sharded_param.is_meta:
+            padded_sharded_param = padded_sharded_param.cpu()
+            if self.pin_memory:
+                padded_sharded_param = padded_sharded_param.pin_memory()
+        self._sharded_param_data = padded_sharded_param.view(-1)
+        self.sharded_param = nn.Parameter(
+            self.to_sharded_dtensor(padded_sharded_param[: sharded_param.size(0)])
+        )
+        self.sharded_param.requires_grad_(param.requires_grad)
+        # Let `param_data` be freed normally when its ref count reaches 0 when
+        # the `fully_shard` call returns to allow provided parameters to alias
+        self._setattr_on_modules(self.sharded_param)
+        self.sharded_state = ShardedState.SHARDED
+
+    def _init_sharded_post_forward_param_metadata(self, param: torch.Tensor) -> None:
+        mesh_info = self.post_forward_mesh_info
+        assert mesh_info is not None  # mypy
+        param_data = param._local_tensor if isinstance(param, DTensor) else param
+        chunks = _chunk_with_empty(param_data, mesh_info.shard_mesh_size, dim=0)
+        self.sharded_post_forward_size = _get_dim0_chunked_size(
+            chunks[mesh_info.shard_mesh_rank], param_data.size()
+        )
+        self.contiguous_sharded_post_forward_stride = make_contiguous_strides_for(
+            self.sharded_post_forward_size
+        )
+
+    def init_dtype_attrs(self, mp_policy: MixedPrecisionPolicy):
+        param_dtype, reduce_dtype = (mp_policy.param_dtype, mp_policy.reduce_dtype)
+        self.orig_dtype = self.sharded_param.dtype
+        # Clamp `param_dtype` to `None` if no casting is required
+        if param_dtype == self.orig_dtype:
+            param_dtype = None
+        self.param_dtype = param_dtype
+        self.reduce_dtype = reduce_dtype
+        # None indicates that the mixed precision is not enabled
+
+    def _init_extensions(self) -> None:
+        inner_tensor = self._sharded_local_tensor
+        has_fsdp_pre_all_gather = hasattr(inner_tensor, "fsdp_pre_all_gather")
+        has_fsdp_post_all_gather = hasattr(inner_tensor, "fsdp_post_all_gather")
+        if has_fsdp_pre_all_gather != has_fsdp_post_all_gather:
+            raise AssertionError(
+                "Both fsdp_pre_all_gather and fsdp_post_all_gather should be defined "
+                f"if using all-gather extensions: {inner_tensor}"
+            )
+        if has_fsdp_pre_all_gather:
+            if self.padded_sharded_param_size != self._sharded_local_tensor.size():
+                raise NotImplementedError(
+                    "FSDP all-gather extensions require even sharding on dim-0.\n"
+                    f"{self._orig_size} is not divisible by FSDP world size {self.mesh_info.mesh.size()}."
+                )
+            self._extensions_data = ExtensionsData()
+        self._unsharded_inner_tensors: List[torch.Tensor] = []
+
+    def init_all_gather_outputs(
+        self,
+        all_gather_input_numels: List[int],
+        all_gather_input_dtypes: List[torch.dtype],
+        world_size: int,
+        device: torch.device,
+        force_recreate: bool = False,
+    ):
+        if not force_recreate and len(self.all_gather_outputs) > 0:
+            return  # already initialized
+        self.all_gather_outputs = [
+            torch.empty(torch.Size([numel * world_size]), dtype=dtype, device=device)
+            for numel, dtype in zip(all_gather_input_numels, all_gather_input_dtypes)
+        ]
+
+    def init_unsharded_param(self):
+        """
+        [Note: Invariants for torch.compile Traceable FSDP2]
+        1. Under compile, we always re-populate the content of `self._unsharded_param`
+           per AllGather using the slow path.
+        2. Under compile, we always recreate `self.all_gather_outputs` per AllGather.
+           This is to ensure the buffer creation is internal to the graph and
+           avoid `self.all_gather_outputs` being captured as a graph input.
+        3. Under compile, at the end of `free_unsharded_param()`, we always clean up
+           `self.all_gather_outputs` and `self._unsharded_inner_tensors`,
+           to avoid them being captured as graph output.
+
+        With these invariants, only these tensors will be inputs to the graph:
+        - Sharded parameters
+        - Placeholders for the `self._unsharded_param` nn.Parameter
+        """
+        if not ca.compiled_autograd_enabled and hasattr(
+            self, "_unsharded_param"
+        ):  # after the 1st all-gather
+            inner_tensor = self._sharded_local_tensor
+            if not hasattr(inner_tensor, "fsdp_post_all_gather"):
+                return  # already initialized
+            for tensor in self._unsharded_inner_tensors:
+                alloc_storage(tensor)
+            all_gather_outputs = self._unflatten_all_gather_outputs()
+            inner_tensor.fsdp_post_all_gather(
+                all_gather_outputs,
+                self._extensions_data.all_gather_metadata,
+                self.param_dtype or self.orig_dtype,
+                out=self._unsharded_param,
+            )
+            self._extensions_data.clear()
+            return
+        inner_tensor = self._sharded_local_tensor
+        if not ca.compiled_autograd_enabled and hasattr(
+            inner_tensor, "fsdp_post_all_gather"
+        ):
+            all_gather_outputs = self._unflatten_all_gather_outputs()
+            (
+                unsharded_tensor,
+                self._unsharded_inner_tensors,
+            ) = inner_tensor.fsdp_post_all_gather(
+                all_gather_outputs,
+                self._extensions_data.all_gather_metadata,
+                self.param_dtype or self.orig_dtype,
+            )
+            self._extensions_data.clear()
+        else:
+            # For the default path (no post-all-gather), the all-gather output
+            # gives the unsharded parameter data directly
+            assert len(self.all_gather_outputs) == 1, f"{len(self.all_gather_outputs)}"
+            unsharded_tensor = self.all_gather_outputs[0]
+        unsharded_param = torch.as_strided(
+            unsharded_tensor,
+            self._orig_size,
+            self._contiguous_orig_stride,
+            storage_offset=0,
+        )
+        if self.is_dtensor:
+            unsharded_param = _from_local_no_grad(unsharded_param, self._tp_spec)
+        if hasattr(self, "_unsharded_param"):
+            assert ca.compiled_autograd_enabled
+            with torch.no_grad(), torch.autograd._unsafe_preserve_version_counter(
+                self._unsharded_param
+            ):
+                torch.ops.fsdp.set_.default(self._unsharded_param, unsharded_param)
+        else:
+            self._unsharded_param = nn.Parameter(
+                unsharded_param, requires_grad=self.sharded_param.requires_grad
+            )
+
+    def _unflatten_all_gather_outputs(self) -> Tuple[torch.Tensor, ...]:
+        return tuple(
+            t.view(-1, *s[1:])
+            for t, s in zip(
+                self.all_gather_outputs, self._extensions_data.all_gather_input_sizes
+            )
+        )
+
+    def to_sharded(self) -> None:
+        self._setattr_on_modules(self.sharded_param)
+        self.free_unsharded_param()
+        self.sharded_state = ShardedState.SHARDED
+
+    def to_sharded_post_forward(self) -> None:
+        if self.is_dtensor:
+            raise NotImplementedError(
+                "Resharding to smaller mesh with TP is not supported yet"
+            )
+        self._assert_in_states(ShardedState.UNSHARDED)
+        assert self.post_forward_mesh_info is not None  # mypy
+        assert len(self.all_gather_outputs) == 1
+        shard_world_size = self.post_forward_mesh_info.shard_mesh_size
+        if (numel := self.all_gather_outputs[0].numel()) % shard_world_size != 0:
+            _raise_assert_with_print(
+                f"All-gather output size ({numel}) must be divisible by the shard "
+                f"world size ({shard_world_size})"
+            )
+        shard_rank = self.post_forward_mesh_info.shard_mesh_rank
+        sharded_numel = numel // shard_world_size
+        self._sharded_post_forward_param_data = (
+            self.all_gather_outputs[0].narrow(
+                0, sharded_numel * shard_rank, sharded_numel
+            )
+        ).clone()  # clone to be able to free all-gather output
+        sharded_post_forward_tensor = torch.as_strided(
+            self._sharded_post_forward_param_data,
+            size=self.sharded_post_forward_size,
+            stride=self.contiguous_sharded_post_forward_stride,
+            storage_offset=0,
+        )
+        self._sharded_post_forward_param = nn.Parameter(
+            self.to_sharded_post_forward_dtensor(sharded_post_forward_tensor)
+        )
+        self._setattr_on_modules(self._sharded_post_forward_param)
+        self.free_unsharded_param()
+        self.sharded_state = ShardedState.SHARDED_POST_FORWARD
+
+    def to_unsharded(self) -> None:
+        # Assume that the data has been allocated and all-gathered
+        set_requires_grad_if_needed(self.sharded_param, self._unsharded_param)
+        self._setattr_on_modules(self._unsharded_param)
+        if self.sharded_state == ShardedState.SHARDED_POST_FORWARD:
+            # The data is allocated in the default stream via the post-forward
+            # reshard and must be kept alive for the next all-gather copy-in.
+            # Since we call this method after the copy-out, the data's lifetime
+            # is ensured without further synchronization.
+            self._sharded_post_forward_param = None
+            self._sharded_post_forward_param_data = None  # free
+        self.sharded_state = ShardedState.UNSHARDED
+
+    def _setattr_on_modules(self, param: nn.Parameter) -> None:
+        unsafe_setattr_param(
+            self._module_info.module, self._module_info.param_name, param
+        )
+        for shared_module, shared_param_name in zip(
+            self._module_info.shared_modules, self._module_info.shared_param_names
+        ):
+            unsafe_setattr_param(shared_module, shared_param_name, param)
+
+    def to_sharded_dtensor(self, tensor: torch.Tensor) -> DTensor:
+        """
+        Converts a local tensor representing either the sharded parameter or
+        sharded gradient to DTensor.
+        """
+        if tensor.shape != self.sharded_size:
+            _raise_assert_with_print(
+                f"Expects size {self.sharded_size} but got {tensor.shape}"
+            )
+        return _from_local_no_grad(
+            tensor,
+            self._sharding_spec,
+        )
+
+    def to_sharded_post_forward_dtensor(self, tensor: torch.Tensor) -> DTensor:
+        if tensor.shape != self.sharded_post_forward_size:
+            _raise_assert_with_print(
+                f"Expects size {self.sharded_post_forward_size} but got {tensor.shape}"
+            )
+        assert isinstance(self.post_forward_mesh_info, HSDPMeshInfo)
+        # TODO: Prefer this DTensor to be read-only and generalize the
+        # placement once we support TP.
+        post_forward_sharding_spec = DTensorSpec(
+            self.post_forward_mesh_info.mesh,
+            (Replicate(), Shard(0)),
+            tensor_meta=self._sharding_spec.tensor_meta,
+        )
+        return _from_local_no_grad(tensor, post_forward_sharding_spec)
+
+    def to_accumulated_grad_if_needed(self) -> None:
+        # Access `_unsharded_param` to bypass the sharded state check since we
+        # prefer to reshard before upcasting the gradient to save memory
+        if (
+            self.reduce_dtype is None
+            or self._unsharded_param.grad is None
+            or self._unsharded_param.grad.dtype == self.reduce_dtype
+        ):
+            return
+        unsharded_grad = self._unsharded_param.grad
+        self._unsharded_param.grad = None
+        self.unsharded_accumulated_grad = unsharded_grad.to(self.reduce_dtype)
+
+    def accumulate_unsharded_grad_if_needed(self) -> None:
+        if (
+            self.unsharded_accumulated_grad is not None
+            and self.unsharded_param.grad is not None
+        ):
+            self.unsharded_accumulated_grad += self.unsharded_param.grad
+            self.unsharded_param.grad = None
+
+    def alloc_all_gather_outputs(self) -> None:
+        for tensor in self.all_gather_outputs:
+            alloc_storage(tensor)
+
+    def free_unsharded_param(self) -> None:
+        for tensor in itertools.chain(
+            self.all_gather_outputs, self._unsharded_inner_tensors
+        ):
+            free_storage(tensor)
+        if ca.compiled_autograd_enabled:
+            self.all_gather_outputs = []
+            self._unsharded_inner_tensors = []
+
+    @property
+    def all_gather_inputs(self) -> List[torch.Tensor]:  # 1D
+        self._assert_in_states(ShardedState.SHARDED, ShardedState.SHARDED_POST_FORWARD)
+        if self.sharded_state == ShardedState.SHARDED:
+            if not ca.compiled_autograd_enabled and hasattr(
+                self._sharded_local_tensor, "fsdp_pre_all_gather"
+            ):
+                sharded_local_tensor = self._sharded_local_tensor
+                if self.offload_to_cpu:
+                    sharded_local_tensor = sharded_local_tensor.to(
+                        self.device, non_blocking=True
+                    )
+                (
+                    all_gather_inputs,
+                    self._extensions_data.all_gather_metadata,
+                ) = sharded_local_tensor.fsdp_pre_all_gather(self.mesh_info.mesh)
+                self._extensions_data.all_gather_input_sizes = [
+                    t.size() for t in all_gather_inputs
+                ]
+                return [t.view(-1) for t in all_gather_inputs]
+            sharded_param_data = self._sharded_param_data
+            if self.offload_to_cpu:
+                sharded_param_data = sharded_param_data.to(
+                    self.device, non_blocking=True
+                )
+            return [_to_dtype_if_needed(sharded_param_data, self.param_dtype)]
+        elif self.sharded_state == ShardedState.SHARDED_POST_FORWARD:
+            if not ca.compiled_autograd_enabled and hasattr(
+                self._sharded_local_tensor, "fsdp_pre_all_gather"
+            ):
+                raise NotImplementedError
+            all_gather_input = _to_dtype_if_needed(
+                cast(torch.Tensor, self._sharded_post_forward_param_data),
+                self.param_dtype,
+            )
+            return [all_gather_input]
+        return [torch.empty(0)]  # mypy
+
+    @property
+    def unsharded_param(self) -> nn.Parameter:  # ND
+        self._assert_in_states(ShardedState.UNSHARDED)
+        return self._unsharded_param
+
+    @property
+    def unsharded_grad_data(self) -> torch.Tensor:
+        grad = self.unsharded_param.grad
+        assert grad is not None, "Expects unsharded_param.grad to not be None"
+        return self._get_grad_inner_tensor(grad)
+
+    @property
+    def unsharded_accumulated_grad_data(self) -> torch.Tensor:
+        grad = self.unsharded_accumulated_grad
+        assert grad is not None, "Expects unsharded_accumulated_grad to not be None"
+        return self._get_grad_inner_tensor(grad)
+
+    def _get_grad_inner_tensor(self, grad: torch.Tensor) -> torch.Tensor:
+        if self.is_dtensor:
+            if isinstance(grad, AsyncCollectiveTensor):
+                grad = grad.wait()
+            assert isinstance(grad, DTensor), f"{type(grad)}"
+            if any(pl.is_partial() for pl in grad.placements):
+                placements = [
+                    Replicate() if pl.is_partial() else pl for pl in grad.placements
+                ]
+                grad = grad.redistribute(placements=placements)
+            grad = grad._local_tensor
+        return grad
+
+    @property
+    def _sharded_local_tensor(self) -> torch.Tensor:
+        return cast(DTensor, self.sharded_param)._local_tensor
+
+    def _assert_in_states(self, *states: ShardedState) -> None:
+        if self.sharded_state not in states:
+            _raise_assert_with_print(
+                f"Expects to be in one of {states}, not {self.sharded_state}"
+            )
+
+    def reset_sharded_param(self):
+        # For ops like `nn.Module._apply` or `load_state_dict(assign=True)`
+        # that change the sharded parameter tensor, we may need to re-pad the
+        # sharded local tensor and re-save the reference.
+        module_info = self._module_info
+        new_param = getattr(module_info.module, module_info.param_name)
+        if new_param is not self.sharded_param:
+            if torch.__future__.get_swap_module_params_on_conversion():
+                raise AssertionError(
+                    f"Expects swap_tensors to preserve object but got {new_param} "
+                    f"instead of {self.sharded_param}"
+                )
+            self.sharded_param = new_param
+        local_tensor = new_param._local_tensor
+        if local_tensor.is_meta:
+            return
+        padded_sharded_size = self.padded_sharded_param_size
+        if local_tensor.size() != padded_sharded_size:
+            padded_local_tensor = local_tensor.new_zeros(padded_sharded_size)
+            padded_local_tensor[: local_tensor.size(0)].copy_(local_tensor)
+            local_tensor = padded_local_tensor
+        if self.pin_memory and not local_tensor.is_pinned():
+            local_tensor = local_tensor.cpu().pin_memory()
+        self._sharded_param_data = local_tensor.view(-1)
+        assert isinstance(self.sharded_param, DTensor)  # mypy
+        self.sharded_param._local_tensor = local_tensor[: self.sharded_size[0]]
+
+    def __repr__(self):
+        return f"FSDPParam(fqn={self._param_fqn}, orig_size={self._orig_size})"
+
+
+def alloc_storage(tensor: torch.Tensor) -> None:
+    size = tensor.numel() * tensor.itemsize
+    if (storage := tensor.untyped_storage()).size() != size:
+        storage.resize_(size)
+
+
+def free_storage(tensor: torch.Tensor) -> None:
+    if (storage := tensor.untyped_storage()).size() != 0:
+        storage.resize_(0)
+
+
+# NOTE: These bypass `nn.Module.__setattr__` checks, which incur non-trivial
+# CPU overhead, if the module did not override it. For FSDP, we know we do not
+# need those checks when transitioning between sharded/unsharded parameters.
+def unsafe_setattr_param(
+    module: nn.Module, param_name: str, param: nn.Parameter
+) -> None:
+    if getattr(module.__setattr__, "__func__", None) is nn.Module.__setattr__:
+        module._parameters[param_name] = param
+    else:  # slow path
+        setattr(module, param_name, param)
+
+
+def set_requires_grad_if_needed(
+    src_tensor: torch.Tensor, dst_tensor: torch.Tensor
+) -> None:
+    # Only call `requires_grad_` if needed to avoid the Python <> C++ context
+    # switch overhead
+    if src_tensor.requires_grad != dst_tensor.requires_grad:
+        dst_tensor.requires_grad_(src_tensor.requires_grad)

janus/lib/python3.10/site-packages/torch/distributed/_composable/fully_shard.py

@@ -0,0 +1,131 @@
+# mypy: allow-untyped-decorators
+from typing import Callable, Iterable, Optional, Union
+from typing_extensions import deprecated
+
+import torch
+import torch.distributed as dist
+import torch.nn as nn
+from torch.distributed._composable.contract import contract
+from torch.distributed._composable_state import _get_module_state, _insert_module_state
+from torch.distributed.fsdp._common_utils import _FSDPState
+from torch.distributed.fsdp._dynamo_utils import _annotate_modules_for_dynamo
+from torch.distributed.fsdp._init_utils import (
+    _init_buffer_state,
+    _init_core_state,
+    _init_device_handle,
+    _init_ignored_module_states,
+    _init_param_handle_from_module,
+    _init_prefetching_state,
+    _init_process_group_state,
+    _init_runtime_state,
+    _init_state_dict_state,
+    HYBRID_SHARDING_STRATEGIES,
+)
+from torch.distributed.fsdp._runtime_utils import (
+    _register_post_forward_hook,
+    _register_pre_forward_hook,
+    _register_root_pre_forward_hook,
+)
+from torch.distributed.fsdp._state_dict_utils import _register_all_state_dict_hooks
+from torch.distributed.fsdp._wrap_utils import _auto_wrap
+from torch.distributed.fsdp.api import (
+    BackwardPrefetch,
+    CPUOffload,
+    MixedPrecision,
+    ShardingStrategy,
+)
+from torch.distributed.fsdp.wrap import _Policy
+
+
+@contract(state_cls=_FSDPState)
+@deprecated(
+    "`torch.distributed._composable.fully_shard` is being deprecated. "
+    "You can continue to use the wrapper based FSDP. "
+    "See usage in: https://github.com/pytorch/pytorch/blob/main/torch/distributed/fsdp/fully_sharded_data_parallel.py. "
+    "`torch.distributed._composable.fully_shard` will be removed after PyTorch 2.5.",
+    category=FutureWarning,
+)
+def fully_shard(
+    module: nn.Module,
+    *,
+    process_group: Optional[dist.ProcessGroup] = None,
+    policy: Optional[_Policy] = None,
+    strategy: Optional[ShardingStrategy] = None,
+    mixed_precision: Optional[MixedPrecision] = None,
+    cpu_offload: Optional[CPUOffload] = None,
+    ignored_modules: Optional[Iterable[torch.nn.Module]] = None,
+    device_id: Optional[Union[int, torch.device]] = None,
+    param_init_fn: Optional[Callable[[nn.Module], None]] = None,
+    sync_module_states: bool = False,
+    forward_prefetch: bool = False,
+    ignored_states: Union[
+        Optional[Iterable[torch.nn.Parameter]], Optional[Iterable[torch.nn.Module]]
+    ] = None,
+) -> nn.Module:
+    """Applies ``FullyShardedDataParallel`` (FSDP) semantics to ``module``."""
+    torch._C._log_api_usage_once("torch.distributed.fully_shard")
+    # Enforce the new auto wrap policy
+    if policy is not None and not isinstance(policy, _Policy):
+        raise ValueError(f"Expects a `_Policy` but got {policy}")
+    state = fully_shard.state(module)
+    state = _init_ignored_module_states(state, module, ignored_modules, ignored_states)
+    state = _init_device_handle(state, module, state._ignored_params, device_id)
+    _annotate_modules_for_dynamo(module, state._ignored_modules, True)
+    state = _init_process_group_state(state, process_group, strategy, policy)
+    if policy is not None:
+        root_kwargs = {
+            "process_group": process_group,
+            "strategy": strategy,
+            "mixed_precision": mixed_precision,
+            "cpu_offload": cpu_offload,
+            "ignored_modules": ignored_modules,
+            "device_id": device_id,
+            "param_init_fn": param_init_fn,
+            "sync_module_states": sync_module_states,
+            "forward_prefetch": forward_prefetch,
+            "ignored_states": ignored_states,
+        }
+        if strategy in HYBRID_SHARDING_STRATEGIES:
+            root_kwargs["process_group"] = (state.process_group, state._inter_node_pg)
+        _auto_wrap(
+            module,
+            policy,
+            state._ignored_modules,
+            state._ignored_params,
+            root_kwargs,
+            fully_shard,
+        )
+    state = _init_core_state(
+        state,
+        strategy or ShardingStrategy.FULL_SHARD,
+        mixed_precision,
+        cpu_offload,
+        limit_all_gathers=True,
+        use_orig_params=True,
+        backward_prefetch_limit=1,
+        forward_prefetch_limit=1,
+    )
+    state = _init_runtime_state(state)
+    state = _init_prefetching_state(
+        state, BackwardPrefetch.BACKWARD_PRE, forward_prefetch=forward_prefetch
+    )
+    state = _init_buffer_state(state, module)
+    state = _init_param_handle_from_module(
+        state, module, device_id, param_init_fn, sync_module_states
+    )
+    state = _init_state_dict_state(state)
+    _register_all_state_dict_hooks(state)
+    _register_pre_forward_hook(state, module)
+    _register_post_forward_hook(state, module)
+    _register_root_pre_forward_hook(state, module)  # prepend last
+    # Always insert the state for the passed-in module even if it has no
+    # managed parameters, in which case it has no handles and does not appear
+    # in `_fully_sharded_module_to_handles`
+    _insert_module_state(module, state)
+    for submodule in module.modules():
+        if (
+            submodule in state._fully_sharded_module_to_handle
+            and _get_module_state(submodule) is None
+        ):
+            _insert_module_state(submodule, state)
+    return module

janus/lib/python3.10/site-packages/torch/distributed/_tools/runtime_estimator.py

@@ -0,0 +1,527 @@
+# Owner(s): ["module: unknown"]
+import math
+import os
+from collections import defaultdict
+from typing import Any, Callable, Dict, List, Set, Tuple
+from typing_extensions import Self
+
+import torch
+import torch.utils._pytree as pytree
+from torch._guards import active_fake_mode
+from torch._inductor.utils import get_device_tflops, get_gpu_dram_gbps
+from torch._subclasses.fake_tensor import FakeTensorMode
+from torch.distributed._tools.mod_tracker import ModTracker
+from torch.utils._mode_utils import no_dispatch
+from torch.utils._python_dispatch import TorchDispatchMode
+from torch.utils.flop_counter import flop_registry
+
+
+aten = torch.ops.aten
+
+# This value is hard-coded here:
+# https://github.com/pytorch/pytorch/blob/5fba5d83f0703ff8077ab65448a998e9ad6598fd/c10/cuda/CUDACachingAllocator.cpp#L117
+_PYTORCH_MIN_ALLOCATE = (
+    2**9 if int(os.environ.get("PYTORCH_NO_CUDA_MEMORY_CACHING", 0)) == 0 else 1
+)
+
+# No fall-back kernel needed/exists for view ops
+_VIEW_OPS = {
+    aten.lift_fresh,
+    aten.t,
+    aten.transpose,
+    aten.view,
+    aten.detach,
+    aten._unsafe_view,
+    aten.split,
+    aten.adjoint,
+    aten.as_strided,
+    aten.diagonal,
+    aten.expand,
+    aten.expand_as,
+    aten.movedim,
+    aten.permute,
+    aten.select,
+    aten.squeeze,
+    aten.mT,
+    aten.mH,
+    aten.real,
+    aten.imag,
+    aten.view_as,
+    aten.unflatten,
+    aten.unfold,
+    aten.unbind,
+    aten.unsqueeze,
+    aten.vsplit,
+    aten.hsplit,
+    aten.split_with_sizes,
+    aten.swapaxes,
+    aten.swapdims,
+    aten.chunk,
+}
+# We can ignore benchmarking tensor create ops
+_CREATE_OPS = {
+    aten.randint,
+    aten.randn,
+    aten.rand,
+    aten.randn_like,
+    aten.rand_like,
+    aten.randint_like,
+    aten.arange,
+    aten.ones_like,
+    aten.zeros_like,
+}
+
+_IGNORE_OPS = _VIEW_OPS | _CREATE_OPS
+
+__all__ = ["RuntimeEstimator"]
+
+
+class RuntimeEstimator(TorchDispatchMode):
+    """
+    Estimates the GPU runtime in milliseconds using various estimation methods under the ``FakeTensorMode``.
+
+    This class provides a ``TorchDispatchMode`` based context manager that can be used to estimate the eager
+    runtime of PyTorch functions. It supports two estimation modes, benchmarking (`operator-level-benchmark`) and
+    roofline cost modeling (`operator-level-cost-model`).
+    For modules executed under this context manager, it agggregates the forward and backward operation runtimes
+    and also records their execution orders.
+
+    Attributes:
+        mod_runtimes (Dict[str, Dict[str, float]]): A dictionary of module runtimes. The key to the outer dictionary
+            is the fully qualified name (FQN) of the module. For each module the forward and backward runtimes of the
+            operations are aggregated in the inner dictionary keyed by 'fw' and 'bw'.
+        mod_fw_pre_order (List[str]): List of module FQNs in pre-forward execution order.
+        mod_bw_pre_order (List[str]): List of module FQNs in pre-backward execution order.
+        mod_fw_post_order (List[str]): List of module FQNs in post-forward execution order.
+        mod_bw_post_order (List[str]): List of module FQNs in post-backward execution order.
+        total_runtime (float): The total estimated runtime in milliseconds.
+
+    Note:
+        1) The benchmarking estimate mode will execute kernels on GPU and assumes that every operation can run in
+            isolation without causing an OOM error. It is also designed to be used only under ``FakeTensorMode``.
+        2) Currently wrapper tensor sub-classes such as ``DTensor`` won't produce correct estimates. We plan to support
+            them in future PRs.
+        3) We only estimate the compute time, if your code has communication, it will not be considered. Again, we will
+            support this in future PRs.
+
+    Example usage:
+
+        .. code-block:: python
+
+            runtime_estimator = RuntimeEstimator()
+            with FakeTensorMode():
+                module = ...
+                optimizer = ...
+                inp = ...
+                with runtime_estimator(estimate_mode_type="operator-level-cost-model"):
+                    loss = module(inp)
+                    loss.backward()
+                    optimizer.step()
+                    optimizer.zero_grad()
+                runtime_estimator.display_modulewise_stats()
+    """
+
+    _float_types: Set[torch.dtype] = {
+        torch.float16,
+        torch.bfloat16,
+        torch.float32,
+        torch.float64,
+    }
+    _no_fallback_kernel: Set[torch._ops._OpNamespace] = set()
+    fake_mode: FakeTensorMode
+
+    def __init__(self) -> None:
+        super().__init__()
+        self._estimate: Callable
+        self._estimate_mode_type: str
+        self._mod_tracker = ModTracker()
+        self.mod_runtimes: Dict[str, Dict[str, float]] = defaultdict(
+            lambda: defaultdict(lambda: 0.0)
+        )
+        self.mod_fw_pre_order: List[str] = []
+        self.mod_bw_pre_order: List[str] = []
+        self.mod_fw_post_order: List[str] = []
+        self.mod_bw_post_order: List[str] = []
+        self.total_runtime: float = 0.0
+
+    # Adapted from: https://github.com/pytorch/pytorch/blob/9b902b3ee3bd608a19543362b66bf06c373dd374/torch/_subclasses/fake_tensor.py#L1969  # noqa: PGH004,B950
+    # NB: returns fake tensors
+    @classmethod
+    def _maybe_run_and_benchmark_fallback_kernel(  # type: ignore[no-untyped-def]
+        cls,
+        func,
+        args,
+        kwargs,
+        orig_not_implemented_exception,
+    ):
+        """
+        Runs and benchmarks a fallback kernel for a given function.
+
+        Args:
+            func (Callable): The function to benchmark.
+            args (Tuple): The arguments to pass to the function.
+            kwargs (Dict[str, Any]): The keyword arguments to pass to the function.
+            orig_not_implemented_exception (Exception): The original exception to raise if the fallback kernel
+                is not implemented.
+
+        Returns:
+            Tuple[Any, float]: A tuple containing the result of the function and
+                the mean operation time in milliseconds.
+        """
+        # these should all be supported, just to be safe
+        # avoid fallback for operators which inplace modify metadata
+        # because the input fake tensors would be umodified
+        if torch.Tag.inplace_view in func.tags:  # type: ignore[attr-defined]
+            raise orig_not_implemented_exception
+
+        inp_impls = {}
+        flat_args, args_spec = pytree.tree_flatten((args, kwargs))
+        # Don't use in_kernel_invocation_manager(fake_mode) as we want to do
+        # REAL compute (not with meta device)
+        with no_dispatch():
+
+            def to_real_tensor(e):  # type: ignore[no-untyped-def]
+                if cls.fake_mode.is_our_fake(e):
+                    if e.dtype in cls._float_types:
+                        out = torch.rand_like(e, device=e.fake_device)
+                    else:
+                        out = torch.ones_like(e, device=e.fake_device)
+                    if e.is_sparse:
+                        out._coalesced_(e.is_coalesced())
+                    inp_impls[id(out)] = e
+                    return out
+                return e
+
+            flat_args = [to_real_tensor(a) for a in flat_args]
+            args, kwargs = pytree.tree_unflatten(flat_args, args_spec)
+            r = func(*args, **kwargs)
+            warmup_iters, actual_iters = 2, 3
+            for _ in range(warmup_iters):
+                func(*args, **kwargs)
+            start_event = torch.cuda.Event(enable_timing=True)
+            end_event = torch.cuda.Event(enable_timing=True)
+            start_event.record(torch.cuda.current_stream())
+            for _ in range(actual_iters):
+                func(*args, **kwargs)
+            end_event.record(torch.cuda.current_stream())
+            torch.cuda.synchronize()
+            cuda_time = start_event.elapsed_time(end_event)
+            mean_op_time = cuda_time / actual_iters
+
+        storages = set()
+
+        for e in flat_args:
+            if isinstance(e, torch.Tensor):
+                if not e.is_sparse:
+                    storages.add(e._typed_storage()._cdata)
+
+        # TODO: also check metadata change on inputs
+        # proper aliasing/metadata relationship between outputs and inputs will
+        # not be set up, bc of conversion to device, unless we can reuse an
+        # input impl
+
+        def map_out(e):  # type: ignore[no-untyped-def]
+            if id(e) not in inp_impls and (
+                isinstance(e, torch.Tensor)
+                and not e.is_sparse
+                and e._typed_storage()._cdata in storages
+            ):
+                raise orig_not_implemented_exception
+
+            if isinstance(e, torch.Tensor):
+                if id(e) in inp_impls:
+                    return inp_impls[id(e)]
+                else:
+                    return cls.fake_mode.fake_tensor_converter.from_real_tensor(
+                        cls.fake_mode, e
+                    )
+            else:
+                return e
+
+        return (pytree.tree_map(map_out, r), mean_op_time)
+
+    @classmethod
+    def _benchmark_estimate(cls, func, args, kwargs) -> Tuple[Any, float]:  # type: ignore[no-untyped-def]
+        """
+        Estimates the runtime of a function using benchmarking.
+
+        Args:
+            func: The function to estimate.
+            args: The arguments to pass to the function.
+            kwargs: The keyword arguments to pass to the function.
+            res: The result of the function.
+
+        Returns:
+            Tuple[Any, float]: A tuple containing the result of the function and
+                the mean operation time in milliseconds.
+        """
+        assert isinstance(
+            cls.fake_mode, FakeTensorMode
+        ), "Initialize/Assign FakeTensorMode before using this function"
+        mean_op_time = 0.0
+        if func._overloadpacket not in _VIEW_OPS:
+            try:
+                res, mean_op_time = cls._maybe_run_and_benchmark_fallback_kernel(
+                    func,
+                    args,
+                    kwargs,
+                    NotImplementedError,
+                )
+                return (res, mean_op_time)
+            except NotImplementedError:
+                cls._no_fallback_kernel.add(func._overloadpacket)
+        res = func(*args, **kwargs or {})
+        return (res, mean_op_time)
+
+    # Adapted from: https://github.com/pytorch/pytorch/blob/9b902b3ee3bd608a19543362b66bf06c373dd374/torch/_inductor/scheduler.py#L589  # noqa: PGH004,B950
+    @classmethod
+    def _roofline_estimate(cls, func, args, kwargs) -> Tuple[Any, float]:  # type: ignore[no-untyped-def]
+        """
+        Estimates the runtime of a function using a roofline cost model.
+
+        Args:
+            func: The function to estimate.
+            args: The arguments to pass to the function.
+            kwargs: The keyword arguments to pass to the function.
+            out: The output of the function.
+
+        Returns:
+            Tuple[Any, float]: A tuple containing the result of the function and
+                the mean operation time in milliseconds.
+        """
+        assert (
+            torch.cuda.is_available()
+        ), "Roofline estimation needs to access CUDA capabilities to make estimations"
+
+        def get_num_bytes(t: torch.Tensor) -> int:
+            """
+            Calculates the memory consumption of a tensor.
+
+            Args:
+                t (torch.Tensor): The input tensor.
+
+            Returns:
+                int: The memory consumption of the tensor in bytes.
+            """
+            num_bytes = t.untyped_storage().nbytes()
+            mem_consumed = (
+                math.ceil(num_bytes / _PYTORCH_MIN_ALLOCATE) * _PYTORCH_MIN_ALLOCATE
+            )
+            return mem_consumed
+
+        def get_compute_time(func_packet, args, kwargs, out, out_dtypes) -> float:  # type: ignore[no-untyped-def]
+            """
+            Estimates the compute time of an aten operator.
+
+            Args:
+                func_packet: The operator overload packet.
+                args: The arguments to the operator.
+                kwargs: The keyword arguments to the operator.
+                out: The output of the operator.
+                out_dtypes: The output data types.
+
+            Returns:
+                float: The estimated compute time in nanoseconds.
+            """
+            if func_packet in flop_registry:
+                assert (
+                    len(out_dtypes) == 1
+                ), f"Only support single out dtype got {out_dtypes} for {func_packet}"
+                dtype = out_dtypes.pop()
+                # This actually gives peta-FLOPs/s hence multiply by 1e15 to get the FLOPs/s
+                peak_gpu_flops = get_device_tflops(dtype) * 1e15
+                # We can expect to achieve 75% of theoretical peak flops
+                factor = 0.75
+                peak_empirical_flops = factor * peak_gpu_flops
+                flop_count_func = flop_registry[func_packet]
+                # We divide by a factor of 2 to get the MACs (multiply and accumulate)
+                flop_count = flop_count_func(*args, **kwargs, out_val=out) / 2
+                # We multiply by 1e9 to get the time in nano seconds
+                compute_time = (flop_count / peak_empirical_flops) * 1e9
+                return compute_time
+            return 0.0
+
+        def get_transfer_time(flat_args_kwargs, flat_outs) -> float:  # type: ignore[no-untyped-def]
+            """
+            Estimates the memory transfer time of input and output tensors.
+
+            Args:
+                flat_args_kwargs (List[torch.Tensor]): The flat list of arguments and keyword arguments.
+                flat_outs (List[torch.Tensor]): The flat list of outputs.
+
+            Returns:
+                float: The estimated memory transfer time in nanoseconds.
+            """
+            gpu_memory_bandwidth = get_gpu_dram_gbps()
+            read_bytes = sum(
+                get_num_bytes(t)
+                for t in flat_args_kwargs
+                if isinstance(t, torch.Tensor)
+            )
+            write_bytes = sum(
+                get_num_bytes(t) for t in flat_outs if isinstance(t, torch.Tensor)
+            )
+            counted_bytes = read_bytes + write_bytes
+            # The GPU memory bandwidth is in GB/s so the transfer time is in nanoseconds
+            transfer_time = counted_bytes / gpu_memory_bandwidth
+            return transfer_time
+
+        # Roofline Cost Model Explanation
+
+        # The roofline cost model estimates the execution time of an operator based on
+        # the device's empirical maximum FLOPs/sec (pi) and device DRAM bandwidth (beta).
+
+        # Variables:
+        # - pi: Maximum empirical FLOPs/sec of the device
+        # - beta: Maximum empirical device DRAM bandwidth (bytes/sec) of the device
+        # - I: Arithmetic intensity of the operator (FLOPs/bytes)
+        # - op_flops: FLOPs required by the operator
+        # - op_bytes: Bytes transferred to and from DRAM for the operator
+
+        # Calculation Steps:
+        # 1. Calculate arithmetic intensity: I = op_flops / op_bytes
+        # 2. Calculate estimated FLOPs/sec: est_flops_sec = min(pi, beta * I)
+        # 3. Calculate estimated operator time: estimated_op_time = op_flops / est_flops_sec
+        #    This simplifies to: estimated_op_time = max(op_flops / pi, op_flops / (beta * I))
+        #    Further simplifying: estimated_op_time = max(op_flops / pi, op_bytes / beta)
+
+        # Simplified Formulas:
+        # - compute_time = op_flops / pi
+        # - transfer_time = op_bytes / beta
+        # - estimated_op_time = max(compute_time, transfer_time)
+
+        kwargs = kwargs if kwargs else {}
+        out = func(*args, **kwargs)
+        op_time = 0.0
+        func_packet = func._overloadpacket
+        if func_packet not in _IGNORE_OPS:
+            flat_args_kwargs, args_spec = pytree.tree_flatten((args, kwargs))
+            flat_outs, out_spec = pytree.tree_flatten(out)
+            transfer_time = get_transfer_time(flat_args_kwargs, flat_outs)
+
+            out_dtypes = {
+                t.dtype
+                for t in flat_outs
+                if isinstance(t, torch.Tensor) and t.dtype in cls._float_types
+            }
+
+            args, kwargs = pytree.tree_unflatten(flat_args_kwargs, args_spec)
+            out = pytree.tree_unflatten(flat_outs, out_spec)
+
+            compute_time = get_compute_time(func_packet, args, kwargs, out, out_dtypes)
+            # We get the estimated time as the max of the transfer time and
+            # compute time. We divide by 1e6 to get the time in ms
+            op_time = max(transfer_time, compute_time) / 1e6
+
+        return (out, op_time)
+
+    def display_modulewise_stats(self, depth: int = 2) -> None:
+        """
+        Displays module-wise statistics collected by ``RuntimeEstimator``.
+
+        Prints the pre-forward and pre-backward execution orders.
+        Displays the module-wise forward and backward runtimes in milliseconds.
+
+        Args:
+            depth (int): The maximum depth of module hierarchy to display (default to 2).
+        """
+        print("Pre-Forward Execution Order: ")
+        for mod_fqn in self.mod_fw_pre_order:
+            mod_depth = mod_fqn.count(".") + 1
+            if mod_depth > depth:
+                continue
+            print(mod_fqn)
+        print("Pre-Backward Execution Order: ")
+        for mod_fqn in self.mod_bw_pre_order:
+            mod_depth = mod_fqn.count(".") + 1
+            if mod_depth > depth:
+                continue
+            print(mod_fqn)
+        for mod_fqn, runtimes in self.mod_runtimes.items():
+            mod_depth = mod_fqn.count(".") + 1
+            if mod_depth > depth:
+                continue
+            print(
+                f"{mod_fqn} fw: {runtimes.get('fw', 0.0):.3f}ms bw: {runtimes.get('bw', 0.0):.3f}ms"
+            )
+
+    def __torch_dispatch__(self, func, types, args=..., kwargs=None):  # type: ignore[no-untyped-def]
+        # TODO: @sanketpurandare: Flatten tensors by desugaring the tensor subclasses
+        # TODO: @sanketpurandare: Add logic for incorporating communication time
+        res, op_time = self._estimate(func, args, kwargs)
+        for par in self._mod_tracker.parents:
+            if self._mod_tracker.is_bw:
+                self.mod_runtimes[par]["bw"] += op_time
+            else:
+                self.mod_runtimes[par]["fw"] += op_time
+        self.total_runtime += op_time
+        return res
+
+    def __call__(self, estimate_mode_type: str) -> Self:
+        """
+        Sets the estimate mode type.
+
+        Currently supported modes:
+            - "operator-level-benchmark": Estimates runtime using operator benchmarking.
+            - "operator-level-cost-model": Estimates runtime using roofline cost model.
+
+        Args:
+            estimate_mode_type (str): The type of estimate mode to use.
+
+        Returns:
+            RuntimeEstimator: The runtime estimator instance.
+
+        Raises:
+            NotImplementedError: If the estimate mode type is not supported.
+        """
+        if estimate_mode_type == "operator-level-benchmark":
+            self._estimate = RuntimeEstimator._benchmark_estimate
+        elif estimate_mode_type == "operator-level-cost-model":
+            self._estimate = RuntimeEstimator._roofline_estimate
+        else:
+            raise NotImplementedError(
+                f"estimate_mode_type {estimate_mode_type} not supported"
+            )
+        self._estimate_mode_type = estimate_mode_type
+        return self
+
+    def __enter__(self) -> Self:
+        fake_mode = active_fake_mode()
+        assert isinstance(
+            fake_mode, FakeTensorMode
+        ), "No FakeTensorMode found, designed to used under FakeTensorMode"
+        RuntimeEstimator.fake_mode = fake_mode
+        self.total_runtime = 0.0
+        self.mod_runtimes = defaultdict(lambda: defaultdict(lambda: 0.0))
+        self.mod_fw_pre_order.clear()
+        self.mod_bw_pre_order.clear()
+        self.mod_fw_post_order.clear()
+        self.mod_bw_post_order.clear()
+        self._mod_tracker.register_user_hooks(
+            pre_fw_hook=lambda mod, inp: self.mod_fw_pre_order.append(
+                self._mod_tracker.get_known_fqn(mod)
+            ),
+            pre_bw_hook=lambda mod, g_out: self.mod_bw_pre_order.append(
+                self._mod_tracker.get_known_fqn(mod)
+            ),
+            post_fw_hook=lambda mod, inp, out: self.mod_fw_post_order.append(
+                self._mod_tracker.get_known_fqn(mod)
+            ),
+            post_bw_hook=lambda mod, g_inp: self.mod_bw_post_order.append(
+                self._mod_tracker.get_known_fqn(mod)
+            ),
+        )
+        self._mod_tracker.__enter__()
+        super().__enter__()
+        return self
+
+    def __exit__(self, *args: Any) -> None:
+        print(
+            f"Estimated ({self._estimate_mode_type})"
+            f"total_time: {self.total_runtime:.3f} ms"
+        )
+        if len(self._no_fallback_kernel) > 0:
+            print("no_fallback_kernel: ", list(self._no_fallback_kernel))
+        super().__exit__(*args)
+        self._mod_tracker.clear_user_hooks()
+        self._mod_tracker.__exit__()

janus/lib/python3.10/site-packages/torch/distributed/fsdp/_common_utils.py

@@ -0,0 +1,558 @@
+# mypy: allow-untyped-defs
+"""
+This file includes private common utilities for FSDP.
+"""
+import logging
+import traceback
+import warnings
+import weakref
+from enum import auto, Enum
+from functools import partial
+from typing import (
+    Any,
+    Callable,
+    cast,
+    Dict,
+    Generator,
+    Iterable,
+    List,
+    no_type_check,
+    Optional,
+    Set,
+    Tuple,
+    Type,
+    TYPE_CHECKING,
+)
+
+import torch
+import torch.distributed as dist
+import torch.distributed.fsdp._flat_param as flat_param_file
+import torch.nn as nn
+from torch.distributed._composable_state import _get_module_state, _State
+from torch.distributed.algorithms._checkpoint.checkpoint_wrapper import (
+    _CHECKPOINT_PREFIX,
+)
+from torch.distributed.utils import _apply_to_tensors
+from torch.utils._mode_utils import no_dispatch
+
+from .api import (
+    FullOptimStateDictConfig,
+    FullStateDictConfig,
+    OptimStateDictConfig,
+    ShardingStrategy,
+    StateDictConfig,
+    StateDictType,
+)
+
+
+if TYPE_CHECKING:
+    from torch.distributed.device_mesh import DeviceMesh
+    from torch.distributed.fsdp._fsdp_extensions import FSDPExtensions
+
+    from ._flat_param import FlatParamHandle
+
+FSDP_WRAPPED_MODULE = "_fsdp_wrapped_module"
+FSDP_PREFIX = FSDP_WRAPPED_MODULE + "."
+FSDP_FLATTENED = "_fsdp_flattened"
+
+# Save a global mapping from module to its input tensor dtype to be populated
+# during the forward pre-hook and consumed in the forward post-hook when
+# overriding a module's mixed precision
+# NOTE: We currently take the last input tensor's dtype in the case of multiple
+# floating-point input tensors, which may be incorrect. However, since there is
+# not a 1:1 correspondence between input and output tensors, we must use *some*
+# heuristic like this to predict the desired output dtype.
+_MODULE_TO_INP_DTYPE: weakref.WeakKeyDictionary = weakref.WeakKeyDictionary()
+
+
+class _FSDPDeviceHandle:
+    """
+    This is a simple abstraction for FSDP computing devices,
+    which enables custom backends that implement CUDA-like
+    semantics to be integrated with FSDP.
+    """
+
+    def __init__(self, device: torch.device, backend: Any = None):
+        if backend is None:
+            try:
+                self.__backend = getattr(torch, device.type)
+                self.__device = device
+            except AttributeError as exc:
+                raise AttributeError(
+                    f"Device '{device}' does not have a corresponding backend registered as 'torch.{device.type}'."
+                ) from exc
+        else:
+            self.__backend = backend
+
+    @classmethod
+    def from_device(cls, device: torch.device) -> "_FSDPDeviceHandle":
+        """
+        Return a device handle corresponding to the device, and through this handle,
+        operations with the same semantics as CUDA can be performed on the device.
+        Just return torch.cuda if the device is cuda to make attribute-access faster.
+        Custom backend must first register a module with the same name with {device.type} on torch.
+        """
+        if device.type == "cuda":
+            return cast(_FSDPDeviceHandle, torch.cuda)
+        elif device.type == "mtia":
+            return cast(_FSDPDeviceHandle, torch.mtia)
+        return cls(device)
+
+    def __getattr__(self, __name: str) -> Any:
+        try:
+            return getattr(self.__backend, __name)
+        except AttributeError as exc:
+            raise AttributeError(
+                f"Custom backend '{self.__device.type}' not implement 'torch.{self.__device.type}.{__name}'"
+            ) from exc
+
+
+class _UninitializedDeviceHandle(_FSDPDeviceHandle):
+    def __init__(self) -> None:
+        pass
+
+    def __getattribute__(self, __name: str) -> Any:
+        raise RuntimeError("Trying to use an uninitialized device handle.")
+
+
+class _FSDPState(_State):
+    def __init__(self) -> None:
+        # TODO: Move all the attributes to this class to enable typing for
+        # FSDP/fully_shard.
+        self._ignored_modules: Set[nn.Module] = set()
+        self._ignored_params: Set[nn.Parameter] = set()
+        # Buffer names are cleaned (without wrapper prefixes)
+        self._ignored_buffer_names: Set[str] = set()
+        self.process_group: Optional[dist.ProcessGroup] = None
+        self.rank: int = -1
+        self.world_size: int = -1
+        self._device_mesh: Optional[DeviceMesh] = None
+        self.sharding_strategy = ShardingStrategy.FULL_SHARD
+        self._use_orig_params: bool = False
+        self.training_state = TrainingState.IDLE
+        self._unshard_params_ctx: Dict[nn.Module, Generator] = {}
+        self._state_dict_type: StateDictType = StateDictType.FULL_STATE_DICT
+        self._state_dict_config: StateDictConfig = FullStateDictConfig()
+        self._optim_state_dict_config: OptimStateDictConfig = FullOptimStateDictConfig()
+        self._is_root: Optional[bool] = None
+        self._handle: Optional[flat_param_file.FlatParamHandle] = None
+        self._fully_sharded_module_to_handle: Dict[
+            nn.Module, Optional[flat_param_file.FlatParamHandle]
+        ] = {}
+        self.compute_device: Optional[torch.device] = None
+        self._gradient_predivide_factor: int = 0
+        self._gradient_postdivide_factor: int = 0
+        self._comm_hook: Optional[Callable] = None
+        self._comm_hook_state: Optional[Any] = None
+        self._unshard_event: Optional[torch.Event] = None
+        # Abstract device handle for fsdp compute device. For now,
+        # the compute device must implement cuda semantics used by fsdp
+        self._device_handle: _FSDPDeviceHandle = _UninitializedDeviceHandle()
+        # All following attributes should only be used for root states:
+        # Save these static lists to avoid the repeated tree traversals
+        self._all_fsdp_states: List[_FSDPState] = []
+        self._all_handles: List[flat_param_file.FlatParamHandle] = []
+        self._fsdp_extension: Optional[FSDPExtensions] = None
+
+
+def _get_module_fsdp_state(module: nn.Module) -> Optional[_FSDPState]:
+    state = _get_module_state(module)
+    if state is None or not isinstance(state, _FSDPState):
+        return None
+    return state
+
+
+def _get_module_fsdp_state_if_fully_sharded_module(
+    module: nn.Module,
+) -> Optional[_FSDPState]:
+    state = _get_module_fsdp_state(module)
+    if state is None:
+        return None
+    if state == module:  # FullyShardedDataParallel module case.
+        return state
+    if module in state._fully_sharded_module_to_handle:  # fully_shard case.
+        return state
+    return None
+
+
+class TrainingState(Enum):
+    """
+    An enum that indicates the state of a ``FullyShardedDataParallel` instance.
+    """
+
+    IDLE = auto()
+    FORWARD_BACKWARD = auto()
+    SUMMON_FULL_PARAMS = auto()
+
+
+class HandleTrainingState(Enum):
+    """
+    An enum that indicates the state of a ``FlatParamHandle`.
+    """
+
+    IDLE = auto()
+    FORWARD = auto()
+    BACKWARD_PRE = auto()
+    BACKWARD_POST = auto()
+    SUMMON_FULL_PARAMS = auto()
+
+
+def _is_composable(state: _FSDPState):
+    # TODO: This is a temporary hack for differentiate between code paths.
+    return not isinstance(state, nn.Module)
+
+
+@no_type_check
+def _module_handle(state: _FSDPState, module: nn.Module) -> Optional["FlatParamHandle"]:
+    """
+    Returns the ``FlatParamHandle`` s corresponding to ``module``. This is
+    the handle that contains some parameter in ``module``.
+    """
+    if _is_composable(state):
+        # A valid FSDP state may have no managed parameters and hence no
+        # handles, meaning no entry in `_fully_sharded_module_to_handles`
+        if state._handle is None:
+            return None
+        assert (
+            module in state._fully_sharded_module_to_handle
+        ), f"Expects a fully sharded module but got {module} on rank {state.rank}"
+        return state._fully_sharded_module_to_handle[module]
+    else:
+        # NOTE: This assumes `module` is a `FullyShardedDataParallel` instance.
+        return module._handle
+
+
+@no_type_check
+def _has_fsdp_params(state: _FSDPState, module: nn.Module) -> bool:
+    """Returns if ``module`` has parameters managed by FSDP."""
+    return _module_handle(state, module) is not None
+
+
+def _get_sharding_strategy(handle):
+    """
+    Returns the sharding strategy of the handle.
+    """
+    return handle._sharding_strategy if handle else None
+
+
+def clean_tensor_name(tensor_name: str) -> str:
+    """
+    Cleans the parameter or buffer name by removing any module wrapper
+    prefixes.
+    """
+    tensor_name = tensor_name.replace(FSDP_PREFIX, "")
+    # TODO: Explicitly replacing the checkpoint wrapper prefix is not ideal as
+    # it couples `CheckpointWrapper` and FSDP and also does not scale for more
+    # module wrappers.
+    tensor_name = tensor_name.replace(_CHECKPOINT_PREFIX, "")
+    return tensor_name
+
+
+def _set_fsdp_flattened(tensor: torch.Tensor) -> None:
+    """
+    Sets an attribute on ``tensor`` to mark it as flattened by FSDP. This is to
+    avoid re-flattening it during nested construction.
+    """
+    setattr(tensor, FSDP_FLATTENED, True)
+
+
+def _is_fsdp_flattened(tensor: torch.Tensor) -> bool:
+    """Returns if ``tensor`` has been marked as flattened by FSDP."""
+    return getattr(tensor, FSDP_FLATTENED, False)
+
+
+def _named_parameters_with_duplicates(
+    module: nn.Module, **kwargs: Any
+) -> List[Tuple[str, nn.Parameter]]:
+    """
+    This API is required as some modules overwrite `named_parameters()` but do not support
+    `remove_duplicate`.
+    """
+    assert (
+        "remove_duplicate" not in kwargs
+    ), "_named_parameters_with_duplicates cannot be used with `remove_duplicate` argument."
+    kwargs["remove_duplicate"] = False
+    try:
+        ret = list(module.named_parameters(**kwargs))
+    except AssertionError as e:
+        kwargs.pop("remove_duplicate")
+        ret = list(module.named_parameters(**kwargs))
+    return ret
+
+
+def _get_param_to_fqns(
+    model: torch.nn.Module,
+    dedup_shared_params: bool = True,
+) -> Dict[nn.Parameter, List[str]]:
+    """
+    Constructs a mapping from parameter to a list of its \"canonical\" FQNs. Here,
+    we use canonical to mean the fully-qualified name assigned to the parameter
+    based on its position in the original nn.Module hierarchy before any wrapper
+    or parallelism has been applied to it. This is in contrast to FQNs that may be
+    generated after parallelisms or wrappers have been applied to the model.
+
+    Each normal parameter maps to a singleton list containing its FQN, while each
+    ``FlatParameter`` maps to a list of its original parameter FQNs, which may
+    have length greater than one.  All FQNs are prefixed starting from ``model``.
+
+    In the case where FSDP was applied with ``use_orig_params=True``, there should be no
+    ``FlatParameter`` s registered to the model's modules and this mapping will only
+    contain mappings from ``nn.Parameter`` s to singleton FQN lists.
+
+    It is only in the case where FSDP was applied with ``use_orig_params=False`` where
+    a ``FlatParameter`` will be registered in place of the original parameters and there
+    will be mappings from each ``FlatParameter`` to lists of FQNs corresponding to the
+    original parameters.
+
+    Args:
+        model (torch.nn.Module): Root module (which may or may not be a
+            :class:`FullyShardedDataParallel` instance).
+        dedup_shared_params (bool): For shared parameters, if ``True``, only
+            includes the FQNs corresponding to the first encounter of the
+            shared parameter in the module traversal; if ``False``, then
+            includes the FQNs across all encounters. (Default: ``True``)
+    """
+
+    def module_fn(module, prefix, tree_level, param_to_fqns):
+        for param_name, param in _named_parameters_with_duplicates(
+            module, recurse=False
+        ):
+            local_fqns = (
+                param._fqns
+                if isinstance(param, flat_param_file.FlatParameter)
+                else [param_name]
+            )  # prefixed from `module`
+            global_fqns = [
+                clean_tensor_name(prefix + name) for name in local_fqns
+            ]  # prefixed from the top level `model` (i.e. including `prefix`)
+            is_shared_param = param in param_to_fqns
+            if not is_shared_param:
+                param_to_fqns[param] = global_fqns
+            else:
+                if isinstance(param, flat_param_file.FlatParameter):
+                    # DMP overwrites `named_parameters` and skip (advance to
+                    # the next child module) the wrapped_module (e.g.,
+                    # _dmp_wrapped_module and _fsdp_wrapped_module). When a user
+                    # calls `named_child` to traverse the module recursively and
+                    # calls `named_parameters` with `recurse=False`, parameters
+                    # will be traversed more than once.
+                    # This hack is specified designed for DMP + FSDP. We
+                    # overwrite the flat_parameters traversal result to only obtain
+                    # the last one, which happens to be the correct one.
+                    #
+                    # TODO: Remove this hack once DMP + FSDP is not supported.
+                    warnings.warn(
+                        "FlatParameter is being traversed more than once. "
+                        "This case should only happen when using "
+                        "DistributedModelParallel with FullyShardedDataParallel."
+                    )
+                    param_to_fqns[param] = global_fqns
+                elif not dedup_shared_params:
+                    param_to_fqns[param].extend(global_fqns)
+
+    def return_fn(param_to_fqns):
+        return param_to_fqns
+
+    param_to_unflat_param_names: Dict[torch.nn.Parameter, List[str]] = {}
+    return _apply_to_modules(
+        model,
+        module_fn,
+        return_fn,
+        [key for key, _ in _named_parameters_with_duplicates(model)],
+        param_to_unflat_param_names,
+    )
+
+
+@no_type_check
+def _log_post_backward_hook(
+    state: _FSDPState, handle: "FlatParamHandle", logger: logging.Logger
+) -> None:
+    # Under TORCH_DISTRIBUTED_DEBUG=INFO, log the module names this hook fires for.
+    # Below logging of module names this post-bwd hook fires for can help debug certain
+    # cases where hooks don't fire, such as under certain activation checkpoint configs.
+    if state._use_orig_params and handle._debug_level == dist.DebugLevel.INFO:
+        param_fqns = _get_handle_fqns_from_root(state, handle)
+        logger.warning("FSDP firing post-backward hooks for parameters %s", param_fqns)
+
+
+@no_type_check
+def _get_handle_fqns_from_root(
+    state: _FSDPState, handle: "FlatParamHandle"
+) -> Optional[List[str]]:
+    if handle is None:
+        return None
+    param_to_fqn = state._exec_order_data.param_to_fqn
+    handle_params = handle.flat_param._params  # only populated for use_orig_params
+    param_fqns = [
+        fqn for fqn_list in [param_to_fqn[p] for p in handle_params] for fqn in fqn_list
+    ]
+    return param_fqns
+
+
+def _apply_to_modules(
+    root_module: torch.nn.Module,
+    module_fn: Callable,
+    return_fn: Callable,
+    filter_fqns: Optional[List[str]] = None,
+    *args,
+    **kwargs,
+):
+    """
+    Performs a pre-order traversal of the modules in the hierarchy rooted at
+    ``root_module``, applying ``module_fn`` at each module and finally
+    returning a value using ``return_fn``. The traversal constructs the full
+    module prefix name (e.g. "module.submodule." just like in model state dict)
+    and makes that available to ``module_fn``.
+
+    ``filter_fqns`` is used because some module may have its own prefix similar
+    to ``FullyShardedDataParallel`` and the ``named_parameters()`` is overwritten
+    to remove the prefix.
+    """
+
+    def f(module: torch.nn.Module, prefix: str, tree_level: int, *args, **kwargs):
+        # Call the module function before recursing over children (pre-order)
+        module_fn(module, prefix, tree_level, *args, **kwargs)
+        for submodule_name, submodule in module.named_children():
+            if submodule is None:
+                continue
+            new_prefix = prefix + submodule_name + "."
+            new_tree_level = tree_level + 1
+            if filter_fqns is not None:
+                for fqn in filter_fqns:
+                    if fqn.startswith(new_prefix):
+                        break
+                else:
+                    # DMP's named_parameter() will mess up the traversal with
+                    # ``named_children`` + `named_parameter(recurse=False)``.
+                    # This hack is a must to make the traversal work.
+                    # TODO: Remove this hack once DMP + FSDP is not supported.
+                    # It turns out that recursive wrapping may trigger this as
+                    # well.
+                    if (
+                        submodule_name == "_fsdp_wrapped_module"
+                        or submodule_name == "_dmp_wrapped_module"
+                    ):
+                        new_prefix = prefix
+                    elif submodule_name == "module":
+                        new_prefix = prefix
+            f(submodule, new_prefix, new_tree_level, *args, **kwargs)
+
+    f(root_module, "", 0, *args, **kwargs)
+    return return_fn(*args, **kwargs)
+
+
+@no_type_check
+def _assert_in_training_states(
+    state: _FSDPState,
+    training_states: List[TrainingState],
+) -> None:
+    """Asserts that FSDP is in the states ``_training_states``."""
+    # Raise a `ValueError` instead of using `assert` to ensure that these
+    # logical assertions run even if `assert`s are disabled
+    if state.training_state not in training_states:
+        msg = (
+            f"expected to be in states {training_states} but current state is "
+            f"{state.training_state}"
+        )
+        # Print the error on rank 0 in case this is called in the backward pass
+        if state.rank == 0:
+            if isinstance(state, nn.Module):
+                print(f"Asserting FSDP instance is: {state}")
+            print(f"ERROR: {msg}")
+            traceback.print_stack()
+        raise ValueError(msg)
+
+
+def _get_root_modules(modules: Set[nn.Module]) -> Set[nn.Module]:
+    """
+    Returns:
+        Set[nn.Module]: The subset of ``modules`` that are root modules (i.e.
+        parent-less) with respect to the modules in the set itself. In other
+        words, these are the modules in ``modules`` that are not the child of
+        any other module in ``modules``.
+    """
+    root_modules: Set[nn.Module] = set()
+    module_to_submodules = {module: set(module.modules()) for module in modules}
+    for candidate_module in modules:
+        is_root_module = True
+        for module, submodules in module_to_submodules.items():
+            is_child_module = (
+                candidate_module is not module and candidate_module in submodules
+            )
+            if is_child_module:
+                is_root_module = False
+                break
+        if is_root_module:
+            root_modules.add(candidate_module)
+    return root_modules
+
+
+def _override_module_mixed_precision(
+    root: torch.nn.Module,
+    module_classes_to_override: Iterable[Type[nn.Module]],
+    wrap_override_dict: Dict[str, Any] = {"mixed_precision": None},  # noqa: B006
+) -> Set[Type[nn.Module]]:
+    module_classes_to_override = tuple(set(module_classes_to_override))
+    # Return a set of the actually overridden module classes
+    overridden_module_classes: Set[Type[nn.Module]] = set()
+    for mod in root.modules():
+        if isinstance(mod, module_classes_to_override):
+            overridden_module_classes.add(type(mod))
+            mod._wrap_overrides = wrap_override_dict  # type: ignore[assignment]
+            # TODO: We need to run this mixed precision ignored module in fp32,
+            # but ensure subsequent modules, that may possibly be running with
+            # mixed precision, still receive the appropriate precision inputs
+            # without user having to adjust mixed precision config too much.
+            # As a result, we attach pre and post forward hooks to up / down
+            # cast. We should revisit this design.
+
+            def cast_fn(
+                dtype: torch.dtype, module: nn.Module, x: torch.Tensor
+            ) -> torch.Tensor:
+                if not torch.is_floating_point(x) or x.dtype == dtype:
+                    return x
+                _MODULE_TO_INP_DTYPE[module] = x.dtype
+                return x.to(dtype)
+
+            def forward_pre_hook(module, args):
+                return _apply_to_tensors(partial(cast_fn, torch.float32, module), args)
+
+            def forward_post_hook(module, args, output):
+                # NOTE: If the forward did not have any floating-point tensors,
+                # then the dtype will not be set for this module, and we do not
+                # upcast the dtype.
+                if module in _MODULE_TO_INP_DTYPE:
+                    old_dtype = _MODULE_TO_INP_DTYPE[module]
+                    return _apply_to_tensors(
+                        partial(cast_fn, old_dtype, module), output
+                    )
+
+            # We intentionally append both of these hooks so that they run after
+            # all other hooks.
+            mod.register_forward_pre_hook(forward_pre_hook, prepend=False)
+            mod.register_forward_hook(forward_post_hook, prepend=False)
+    return overridden_module_classes
+
+
+def _no_dispatch_record_stream(tensor: torch.Tensor, stream: torch.Stream) -> None:
+    # FIXME record_stream doesn't work with non-cuda/mtia tensors
+    if tensor.device.type not in [
+        "cuda",
+        "mtia",
+        torch._C._get_privateuse1_backend_name(),
+    ]:
+        return
+
+    if torch.distributed._functional_collectives.is_torchdynamo_compiling():
+        return
+        # from @ezyang:
+        # The no_dispatch was added in https://github.com/pytorch/pytorch/pull/88014 cc @fegin
+        # Looking over the PR, it looks like this is because we don't actually support Stream arguments
+        # in torch dispatch, so it just chokes.
+        # If Dynamo is able to answer "are there any torch dispatch modes" active (it should answer False),
+        # a better version of this would just be to check if there are any modes before disabling dispatch.
+        # TODO(voz): Extend a dynamo util to answer the above, unify the codepaths here.
+        tensor.record_stream(stream)
+    else:
+        with no_dispatch():
+            tensor.record_stream(stream)

janus/lib/python3.10/site-packages/torch/distributed/fsdp/_dynamo_utils.py

@@ -0,0 +1,46 @@
+# mypy: allow-untyped-defs
+from typing import Set
+
+import torch.nn as nn
+
+
+def _annotate_modules_for_dynamo(
+    module: nn.Module,
+    ignored_modules: Set[nn.Module],
+    use_orig_params: bool,
+):
+    """
+    Annotates the submodules in ``module`` 's tree, except those in
+    ``ignored_modules``, indicating that the submodules are FSDP-managed and
+    saving the ``use_orig_params`` setting passed to the FSDP constructor.
+    """
+    for submodule in module.modules():
+        if submodule not in ignored_modules:
+            """[note: Dynamo treats FSDP wrapped modules as UnspecializedNNModule]
+
+            Dynamo doesn't get to see this instance (FullyShardedDataParallel) during tracing, since
+            it skips tracing all the torch.distributed.fsdp code.
+                - Why? Running the FSDP code eagerly avoids lots of issues trying to trace complex hooks, and also
+                gets us graph-breaks on FSDP module boundaries which we want anyway for comm ops.
+                - However, we _also_ want dynamo to treat the wrapped module inside FSDP 'unspecially' (*),
+                and we need a way to indicate to dynamo which modules are wrapped by FSDP.
+
+            (*) UnspecializedNNModules in dynamo are traced-through without any assumptions, and with thorough
+            guards.  NNModules otherwise are 'specialized', meaning there is less overhead due to assuming
+            their code is well-behaved.
+
+            One particular issue with specialized NNModules for FSDP is that the
+            views created for orig_params are captured into the compiled graph on the first iteration, and while
+            they are always going to point to the correct flatparameter and give correct results, their order
+            of creation influences the order of backward execution, preventing overlap of comm and computation
+            during backward.  We need to _use_ the new parameter views created on each forward iteration, in
+            order for backward to interleave hooks with compute per layer.  UnspecializedNNModule lets us achieve
+            this by capturing the module code more 'functionally' and passing parameters in as inputs each time.
+            """
+            submodule._is_fsdp_managed_module = True  # type: ignore[assignment]
+
+            # Dynamo only supports FSDP with use_orig_params=True.
+            # This is hacky, but I could not think of another way to add an assertion to dynamo
+            # for this, since Dynamo skips all the FSDP code frames and thus can't inspect the
+            # FSDP module directly
+            submodule._fsdp_use_orig_params = use_orig_params  # type: ignore[assignment]

janus/lib/python3.10/site-packages/torch/distributed/fsdp/_init_utils.py

@@ -0,0 +1,1200 @@
+# mypy: allow-untyped-defs
+import collections
+import itertools
+import os
+import warnings
+from typing import (
+    Any,
+    Callable,
+    Deque,
+    Dict,
+    Generator,
+    Iterable,
+    Iterator,
+    List,
+    no_type_check,
+    Optional,
+    Set,
+    Tuple,
+    TYPE_CHECKING,
+    Union,
+)
+
+import torch
+import torch.distributed as dist
+import torch.distributed.fsdp._exec_order_utils as exec_order_utils
+import torch.distributed.fsdp._traversal_utils as traversal_utils
+import torch.distributed.fsdp.fully_sharded_data_parallel as fsdp_file
+import torch.nn as nn
+from torch.distributed.algorithms._comm_hooks import default_hooks
+from torch.distributed.device_mesh import _mesh_resources, DeviceMesh
+from torch.distributed.distributed_c10d import _get_default_group
+from torch.distributed.fsdp._common_utils import (
+    _FSDPDeviceHandle,
+    _FSDPState,
+    _get_module_fsdp_state,
+    _is_fsdp_flattened,
+    _named_parameters_with_duplicates,
+    clean_tensor_name,
+    TrainingState,
+)
+from torch.distributed.fsdp._flat_param import (
+    _FSDP_USE_FULL_PREC_IN_EVAL,
+    FlatParameter,
+    FlatParamHandle,
+    HandleShardingStrategy,
+)
+from torch.distributed.fsdp._limiter_utils import _FreeEventQueue
+from torch.distributed.fsdp.api import (
+    BackwardPrefetch,
+    CPUOffload,
+    FullOptimStateDictConfig,
+    FullStateDictConfig,
+    MixedPrecision,
+    ShardingStrategy,
+    StateDictConfig,
+    StateDictType,
+)
+from torch.distributed.fsdp.wrap import _Policy
+from torch.distributed.tensor.parallel.fsdp import DTensorExtensions
+from torch.distributed.utils import _sync_params_and_buffers
+from torch.utils._python_dispatch import is_traceable_wrapper_subclass
+
+
+if TYPE_CHECKING:
+    from torch.utils.hooks import RemovableHandle
+
+_TORCHDISTX_AVAIL = True
+try:
+    from torchdistx import deferred_init, fake  # type: ignore[import]
+except ImportError:
+    _TORCHDISTX_AVAIL = False
+
+PARAM_BROADCAST_BUCKET_SIZE = int(250 * 1024 * 1024)
+FSDP_SYNCED = "_fsdp_synced"
+# Specification of process groups for hybrid sharding strategies.
+HybridShardProcessGroupType = Tuple[dist.ProcessGroup, dist.ProcessGroup]
+# Overall specification of process group.
+ProcessGroupType = Optional[Union[dist.ProcessGroup, HybridShardProcessGroupType]]
+
+
+# TODO (awgu): Refactor this later
+SHARDING_STRATEGY_MAP = {
+    ShardingStrategy.NO_SHARD: HandleShardingStrategy.NO_SHARD,
+    ShardingStrategy.FULL_SHARD: HandleShardingStrategy.FULL_SHARD,
+    ShardingStrategy.SHARD_GRAD_OP: HandleShardingStrategy.SHARD_GRAD_OP,
+    ShardingStrategy.HYBRID_SHARD: HandleShardingStrategy.HYBRID_SHARD,
+    ShardingStrategy._HYBRID_SHARD_ZERO2: HandleShardingStrategy._HYBRID_SHARD_ZERO2,
+}
+HYBRID_SHARDING_STRATEGIES = [
+    ShardingStrategy.HYBRID_SHARD,
+    ShardingStrategy._HYBRID_SHARD_ZERO2,
+]
+NO_RESHARD_AFTER_FORWARD_STRATEGIES = (
+    ShardingStrategy.SHARD_GRAD_OP,
+    ShardingStrategy._HYBRID_SHARD_ZERO2,
+)
+
+
+# NOTE: Since non-self attributes cannot be type annotated, several attributes
+# on `state` are defined first as local variables before being assigned.
+
+
+@no_type_check
+def _init_process_group_state(
+    state: _FSDPState,
+    process_group: ProcessGroupType,
+    sharding_strategy: ShardingStrategy,
+    policy: Optional[_Policy],
+    device_mesh: Optional[DeviceMesh] = None,
+) -> _FSDPState:
+    if process_group is not None and device_mesh is not None:
+        raise ValueError(
+            "Cannot pass both process_group and device_mesh at the "
+            "same time. Please just pass only one of them."
+        )
+    is_hybrid_strategy = sharding_strategy in HYBRID_SHARDING_STRATEGIES
+    if is_hybrid_strategy:
+        if process_group is None and policy is None and device_mesh is None:
+            # Raise an error here, since this is manual wrapping with no process group
+            # passed in, there is no way to ensure all wrapped FSDP instances use the same
+            # process groups.
+            raise ValueError(
+                f"Manual wrapping with {sharding_strategy} "
+                "requires explicit specification of process group or device_mesh."
+            )
+        else:
+            state = _init_process_group_state_for_hybrid_shard(
+                state, process_group, device_mesh
+            )
+    else:
+        if device_mesh:
+            state._device_mesh = device_mesh
+            state.process_group = device_mesh.get_group(mesh_dim=0)
+        else:
+            state.process_group = (
+                process_group if process_group is not None else _get_default_group()
+            )
+
+    state.rank = state.process_group.rank()
+    state.world_size = state.process_group.size()
+    data_parallel_world_size = state.world_size
+    if is_hybrid_strategy:
+        data_parallel_world_size *= state._inter_node_pg.size()
+    state._gradient_predivide_factor = (
+        default_hooks.DefaultState._get_gradient_predivide_factor(
+            data_parallel_world_size
+        )
+    )
+    state._gradient_postdivide_factor = (
+        data_parallel_world_size / state._gradient_predivide_factor
+    )
+    return state
+
+
+@no_type_check
+def _init_process_group_state_for_hybrid_shard(
+    state: _FSDPState,
+    process_group: ProcessGroupType,
+    device_mesh: DeviceMesh,
+) -> _FSDPState:
+    if device_mesh:
+        if _is_valid_hybrid_shard_device_mesh(device_mesh):
+            state._device_mesh = device_mesh
+            # We currently only allow _inter_node_pg to be the outermost dimension, and the
+            # process_group(intra_node) to be the innermost dimension.
+            state._inter_node_pg = device_mesh.get_group(mesh_dim=0)
+            state.process_group = device_mesh.get_group(mesh_dim=1)
+        else:
+            raise ValueError(
+                f"Expected device_mesh to have ndim=2 but got {device_mesh.ndim}"
+            )
+    elif process_group is None:
+        default_group = _get_default_group()
+        intra_node_group, inter_node_group = _init_intra_and_inter_node_groups(
+            default_group, state._device_handle.device_count()
+        )
+        # we shard across intra-node
+        state.process_group = intra_node_group
+        # save _inter_node_pg to allreduce across.
+        state._inter_node_pg = inter_node_group
+    else:
+        # Check type and assign state.process_group and state._inter_node_pg.
+        if _is_valid_hybrid_shard_pg_type(process_group):
+            # Assuming that user passed in as intra node group and inter node group
+            # as documented.
+            state.process_group, state._inter_node_pg = process_group
+        else:
+            raise ValueError(
+                "Expected process_group to be passed in as either None or "
+                f"Tuple[dist.ProcessGroup, dist.ProcessGroup] but got {type(process_group)}"
+            )
+    # Create state for allreduce
+    state._inter_node_state = _get_default_comm_hook_state(
+        process_group=state._inter_node_pg,
+    )
+    return state
+
+
+@no_type_check
+def _is_valid_hybrid_shard_pg_type(process_group: Any) -> bool:
+    return (
+        isinstance(process_group, tuple)
+        and len(process_group) == 2
+        and all(isinstance(pg, dist.ProcessGroup) for pg in process_group)
+    )
+
+
+@no_type_check
+def _is_valid_hybrid_shard_device_mesh(device_mesh: DeviceMesh) -> bool:
+    return isinstance(device_mesh, DeviceMesh) and device_mesh.ndim == 2
+
+
+@no_type_check
+def _init_intra_node_process_group(num_devices_per_node: int) -> dist.ProcessGroup:
+    """
+    Return a process group across the current node.
+
+    For example, given each row is a distinct node:
+    0  1  2  3  4  5  6  7
+    8  9 10 11 12 13 14 15
+    This API would return an intra-node subgroup across
+    [0, 1, ..., 7] or [8, 9, ..., 15] depending on the process's rank.
+    For example, rank 3 would get [0, 1, ..., 7].
+    """
+    intra_node_subgroup, _ = dist.new_subgroups(num_devices_per_node)
+    return intra_node_subgroup
+
+
+@no_type_check
+def _init_inter_node_process_group(
+    global_process_group: dist.ProcessGroup,
+    num_devices_per_node: int,
+) -> dist.ProcessGroup:
+    """
+    Return an inter-node process group where each contained rank has the same local rank.
+
+    For example, given each row is a distinct node:
+    0  1  2  3  4  5  6  7
+    8  9 10 11 12 13 14 15
+    This API would return inter-node process group [0, 8], [1, 9], [2, 10], and so forth
+    depending on the process's rank. For example, rank 1 would get [1, 9], rank 5
+    would get [5, 13].
+    """
+    # the inter-node pg that is returned
+    inter_node_pg = None
+    sharding_backend = dist.get_backend(global_process_group)
+    world_size = dist.get_world_size(global_process_group)
+    # Assuming fully homogeneous setup
+    num_nodes = world_size // num_devices_per_node
+    my_local_rank = dist.get_rank(global_process_group) % num_devices_per_node
+    for local_rank in range(num_devices_per_node):
+        ranks_for_inter_group = [
+            local_rank + (i * num_devices_per_node) for i in range(num_nodes)
+        ]
+        # every rank always needs to call dist.new_group
+        grp = dist.new_group(ranks=ranks_for_inter_group, backend=sharding_backend)
+        if local_rank == my_local_rank:
+            inter_node_pg = grp
+
+    assert (
+        inter_node_pg is not None
+    ), f"{my_local_rank} expected to assign inter-node pg, but did not"
+    return inter_node_pg
+
+
+def _init_intra_and_inter_node_groups(
+    global_process_group: dist.ProcessGroup,
+    num_devices_per_node: int,
+) -> Tuple[dist.ProcessGroup, dist.ProcessGroup]:
+    """
+    Initialize intra and inter-node process groups and return the ones corresponding to this process's rank.
+
+    This function can be used to initialize process groups for ``HYBRID_SHARD`` or
+    ``_HYBRID_SHARD_ZERO2`` in FSDP.
+    This function assumes each node has an equal number of CUDA-enabled devices.
+    Returns:
+        Tuple[dist.ProcessGroup, dist.ProcessGroup]: Intra and inter-node process group.
+    """
+    return (
+        _init_intra_node_process_group(num_devices_per_node),
+        _init_inter_node_process_group(global_process_group, num_devices_per_node),
+    )
+
+
+@no_type_check
+def _init_ignored_module_states(
+    state: _FSDPState,
+    module: nn.Module,
+    ignored_modules: Optional[Iterable[torch.nn.Module]],
+    ignored_states: Union[
+        Optional[Iterable[torch.nn.Parameter]], Optional[Iterable[torch.nn.Module]]
+    ] = None,
+) -> _FSDPState:
+    if ignored_modules is not None and ignored_states is not None:
+        raise ValueError(
+            "Cannot pass both ignored_modules and ignored_states at the "
+            "same time. Please just pass ignored_states."
+        )
+    ignored_parameters = None
+    passed_as_ignored_states = ignored_states is not None
+    if passed_as_ignored_states:
+        ignored_states_list = list(ignored_states)
+        _check_ignored_states(ignored_states_list, True)
+    else:
+        ignored_states_list = []
+        _check_ignored_states(
+            list(ignored_modules) if ignored_modules is not None else [], False
+        )
+    if len(ignored_states_list) > 0:
+        if isinstance(ignored_states_list[0], nn.Parameter):
+            ignored_parameters = ignored_states_list
+        else:
+            ignored_modules = ignored_states_list
+    state._ignored_modules = _get_ignored_modules(module, ignored_modules)
+    state._ignored_params = _get_ignored_params(
+        module,
+        state._ignored_modules,
+        ignored_parameters,
+    )
+    state._ignored_buffer_names = _get_ignored_buffer_names(
+        module,
+        state._ignored_modules,
+    )
+    # TODO: FSDP's contract for buffers is not well-defined. They are
+    # implicitly ignored for most functionality since they are not sharded;
+    # however, FSDP still imposes some semantics on buffers (e.g. buffer mixed
+    # precision). We should formalize this contract and decide if we need to
+    # compute and store `_ignored_buffers`.
+    return state
+
+
+def _check_ignored_states(
+    ignored_states: List[Any], passed_as_ignored_states: bool
+) -> None:
+    """
+    Check that the ignored states are uniformly parameters or uniformly modules.
+
+    We may remove this check in the future if we permit mixing.
+    """
+    if len(ignored_states) == 0:
+        return
+    if passed_as_ignored_states:
+        all_params = all(isinstance(state, nn.Parameter) for state in ignored_states)
+        all_modules = all(isinstance(state, nn.Module) for state in ignored_states)
+        if not all_params and not all_modules:
+            # Sort for consistent ordering for unit test regex matching
+            sorted_types = sorted({type(state) for state in ignored_states}, key=repr)
+            raise ValueError(
+                "ignored_states expects all nn.Parameter or all nn.Module list "
+                f"elements but got types {sorted_types}"
+            )
+    else:
+        if not all(isinstance(state, nn.Module) for state in ignored_states):
+            sorted_types = sorted({type(state) for state in ignored_states}, key=repr)
+            raise ValueError(
+                "ignored_modules expects nn.Module list elements but got "
+                f"types {sorted_types}"
+            )
+
+
+@no_type_check
+def _init_device_handle(
+    state: _FSDPState,
+    module: nn.Module,
+    ignored_params: Set[nn.Parameter],
+    device_id: Optional[Union[int, torch.device]],
+) -> _FSDPState:
+    """
+    Determine device handle used for initializing FSDP.
+
+    If a device is specified by ``device_id``,
+    then returns device handle corresponds to that device type. Otherwise, If the
+    module is already on a non-CPU device, then the device type is that non-CPU device type.
+    If the module is on CPU or meta, then the device type is the current accelerator device.
+    See the :ref:`Accelerators<accelerators>` for details.
+
+
+    This method will be called once ignored paramters was determined, as the device handle maybe needed
+    for other initialization.
+    """
+    determined_device = None
+    if device_id is not None:
+        determined_device = (
+            device_id
+            if isinstance(device_id, torch.device)
+            else torch.device(device_id)
+        )
+    if determined_device is None:
+        for param in _get_orig_params(module, ignored_params):
+            if param.device.type in {"cpu", "meta"}:
+                continue
+            if determined_device is None:
+                determined_device = param.device
+            else:
+                if param.device.type != determined_device.type:
+                    raise RuntimeError(
+                        f"FSDP does not support modules with different device types "
+                        f"but got params on {determined_device.type} and {param.device.type}"
+                    )
+        determined_device = determined_device or torch._C._get_accelerator()
+        if determined_device.type == "cpu":
+            raise RuntimeError(
+                "FSDP needs a non-CPU accelerator device, but no accelerator device is detected."
+            )
+
+    state._device_handle = _FSDPDeviceHandle.from_device(determined_device)
+    return state
+
+
+@no_type_check
+def _init_buffer_state(
+    state: _FSDPState,
+    module: nn.Module,
+) -> _FSDPState:
+    state._buffer_names = _get_buffer_names(module)
+    # Save a mapping from clean fully-qualified buffer name (starting from
+    # `module`) to its original dtype for restoring that dtype during model
+    # checkpointing when buffer mixed precision is enabled. The names should
+    # be clean since the casting happens in a `summon_full_params()` context.
+    _buffer_name_to_orig_dtype: Dict[str, torch.dtype] = {}
+    for buffer_name, buffer in module.named_buffers():
+        buffer_name = clean_tensor_name(buffer_name)
+        _buffer_name_to_orig_dtype[buffer_name] = buffer.dtype
+    state._buffer_name_to_orig_dtype = _buffer_name_to_orig_dtype
+    return state
+
+
+@no_type_check
+def _init_core_state(
+    state: _FSDPState,
+    sharding_strategy: Optional[ShardingStrategy],
+    mixed_precision: Optional[MixedPrecision],
+    cpu_offload: Optional[CPUOffload],
+    limit_all_gathers: bool,
+    use_orig_params: bool,
+    backward_prefetch_limit: int,
+    forward_prefetch_limit: int,
+) -> _FSDPState:
+    # We clamp the strategy to `NO_SHARD` for world size of 1 since they are
+    # currently functionally equivalent. This may change if/when we integrate
+    # FSDP with MoE.
+    if state.world_size == 1:
+        if sharding_strategy != ShardingStrategy.NO_SHARD:
+            warnings.warn(
+                "FSDP is switching to use `NO_SHARD` instead of "
+                f"{sharding_strategy or ShardingStrategy.FULL_SHARD} since "
+                "the world size is 1."
+            )
+        sharding_strategy = ShardingStrategy.NO_SHARD
+    elif sharding_strategy == ShardingStrategy.NO_SHARD:
+        warnings.warn(
+            "The `NO_SHARD` sharding strategy is deprecated. If having issues, "
+            "please use `DistributedDataParallel` instead.",
+            FutureWarning,
+            # Level 1 is here, level 2 is from `FullyShardedDataParallel`, and
+            # level 3 is from the true caller
+            stacklevel=3,
+        )
+    state.sharding_strategy = sharding_strategy or ShardingStrategy.FULL_SHARD
+    state.mixed_precision = mixed_precision or MixedPrecision()
+    if mixed_precision is not None:
+        torch._C._log_api_usage_once(
+            f"torch.distributed.fsdp.mixed_precision.{str(state.mixed_precision)}"
+        )
+    state._use_full_prec_in_eval = (
+        os.environ.get(_FSDP_USE_FULL_PREC_IN_EVAL, "") == "1"
+    )
+    state.cpu_offload = cpu_offload or CPUOffload()
+    state.limit_all_gathers = limit_all_gathers
+    state._use_orig_params = use_orig_params
+    state.training_state = TrainingState.IDLE
+    state._is_root = None
+    state._free_event_queue = _FreeEventQueue()
+    state._debug_level = dist.get_debug_level()
+    state._exec_order_data = exec_order_utils._ExecOrderData(
+        state._debug_level,
+        backward_prefetch_limit,
+        forward_prefetch_limit,
+    )
+    state._unshard_event = None
+    # Mapping from fully sharded module to the handles it is responsible to
+    # unshard and reshard (see [Note: Fully Sharded Module])
+    _fully_sharded_module_to_handle: Dict[nn.Module, FlatParamHandle] = {}
+    state._fully_sharded_module_to_handle = _fully_sharded_module_to_handle
+    # Invariant: `state.params` contains exactly the `FlatParameter`s of the
+    # handles in `state._handle`
+    _handle: Optional[FlatParamHandle] = None
+    state._handle = _handle
+    params: List[FlatParameter] = []
+    state.params = params
+    return state
+
+
+@no_type_check
+def _init_runtime_state(
+    state: _FSDPState,
+) -> _FSDPState:
+    _root_pre_forward_handles: List[RemovableHandle] = []
+    state._root_pre_forward_handles = _root_pre_forward_handles
+    _pre_forward_handles: List[RemovableHandle] = []
+    state._pre_forward_handles = _pre_forward_handles
+    _post_forward_handles: List[RemovableHandle] = []
+    state._post_forward_handles = _post_forward_handles
+    state._sync_gradients = True
+    state._comm_hook = None
+    state._comm_hook_state = None
+    # Used to prevent running the pre-backward hook multiple times
+    return state
+
+
+@no_type_check
+def _init_prefetching_state(
+    state: _FSDPState,
+    backward_prefetch: BackwardPrefetch,
+    forward_prefetch: bool,
+) -> _FSDPState:
+    state.backward_prefetch = backward_prefetch
+    state.forward_prefetch = forward_prefetch
+    # The data structures use tuples of handles to generalize over the case
+    # where a module's forward involves multiple handles.
+    return state
+
+
+@no_type_check
+def _init_extension(state: _FSDPState, device_mesh: DeviceMesh = None) -> _FSDPState:
+    # TODO: we need to add additional check once we support FSDP + PiPPy.
+    # This check is currently sufficient, since we only support FSDP + TP.
+    root_mesh = _mesh_resources.get_root_mesh(device_mesh)
+    # if a root mesh is not the same as device_mesh,
+    # meaning the device_mesh is sliced out from the root mesh.
+    if device_mesh and root_mesh != state._device_mesh:
+        state._fsdp_extension = DTensorExtensions(state._device_handle)
+    else:
+        # We need to explicilty set _fsdp_extension to None.
+        # Otherwise, we will run into an infinite recursion when getting the attribute.
+        state._fsdp_extension = None
+    return state
+
+
+@no_type_check
+def _init_state_dict_state(state: _FSDPState) -> _FSDPState:
+    state._state_dict_type = StateDictType.FULL_STATE_DICT
+    state_dict_config: StateDictConfig = FullStateDictConfig()
+    state._optim_state_dict_config = FullOptimStateDictConfig()
+    state._state_dict_config = state_dict_config
+    unshard_params_ctx: Dict[nn.Module, Generator] = {}
+    state._unshard_params_ctx = unshard_params_ctx
+
+    return state
+
+
+def _verify_managed_params(module: nn.Module, params: List[nn.Parameter]) -> None:
+    """
+    Verify if the parameters are accepted by FSDP. The only restriction now
+    is that the parameter cannot be a scalar tensor (param.shape == []).
+    """
+    for param in params:
+        if len(param.shape) == 0:
+            param_name = ""
+            for name, param_ in module.named_parameters():
+                if param is param_:
+                    param_name = name
+                    break
+            assert param_name
+            raise ValueError(
+                "FSDP doesn't support salar parameters. "
+                f"Change {param_name} to a 1D tensor with numel equal to 1."
+            )
+
+
+@no_type_check
+def _init_param_handle_from_module(
+    state: _FSDPState,
+    fully_sharded_module: nn.Module,
+    device_id: Optional[Union[int, torch.device]],
+    param_init_fn: Optional[Callable[[nn.Module], None]],
+    sync_module_states: bool,
+) -> _FSDPState:
+    """Initialize a ``FlatParamHandle`` from a module ``fully_sharded_module``."""
+    _check_single_device_module(fully_sharded_module, state._ignored_params, device_id)
+    device_from_device_id = _get_device_from_device_id(
+        device_id, state.rank, state._device_handle
+    )
+    is_meta_module, is_torchdistX_deferred_init = _need_to_materialize_module(
+        fully_sharded_module, state._ignored_params, state._ignored_modules
+    )
+    # Materialize the module if needed
+    if (is_meta_module or is_torchdistX_deferred_init) and param_init_fn is not None:
+        _materialize_with_param_init_fn(
+            fully_sharded_module, param_init_fn, state._ignored_modules
+        )
+    elif is_meta_module:
+        _materialize_meta_module(
+            fully_sharded_module,
+            device_id,
+            state._ignored_modules,
+            state._device_handle,
+        )
+    elif is_torchdistX_deferred_init:
+        deferred_init.materialize_module(
+            fully_sharded_module,
+            check_fn=lambda submodule: _get_module_fsdp_state(submodule) is None
+            and submodule not in state._ignored_modules,
+        )
+
+    ignored_buffers = {
+        buffer
+        for ignored_module in state._ignored_modules
+        for buffer in ignored_module.buffers()
+    }
+
+    _move_module_to_device(
+        fully_sharded_module,
+        state._ignored_params,
+        ignored_buffers,
+        device_from_device_id,
+    )
+    state.compute_device = _get_compute_device(
+        fully_sharded_module,
+        state._ignored_params,
+        device_from_device_id,
+        state.rank,
+        state._device_handle,
+    )
+
+    managed_params = list(_get_orig_params(fully_sharded_module, state._ignored_params))
+    _verify_managed_params(fully_sharded_module, managed_params)
+    if sync_module_states:
+        _sync_module_params_and_buffers(
+            fully_sharded_module, managed_params, state.process_group
+        )
+        if state.sharding_strategy in HYBRID_SHARDING_STRATEGIES:
+            _sync_module_params_and_buffers(
+                fully_sharded_module, managed_params, state._inter_node_pg
+            )
+    _init_param_handle_from_params(state, managed_params, fully_sharded_module)
+    return state
+
+
+@no_type_check
+def _init_param_handle_from_params(
+    state: _FSDPState,
+    params: List[nn.Parameter],
+    fully_sharded_module: nn.Module,
+):
+    if len(params) == 0:
+        return
+    handle = FlatParamHandle(
+        params,
+        fully_sharded_module,
+        state.compute_device,
+        SHARDING_STRATEGY_MAP[state.sharding_strategy],
+        state.cpu_offload.offload_params,
+        state.mixed_precision.param_dtype,
+        state.mixed_precision.reduce_dtype,
+        state.mixed_precision.keep_low_precision_grads,
+        state.process_group,
+        state._use_orig_params,
+        fsdp_extension=state._fsdp_extension,
+    )
+    handle.shard()
+    assert not state._handle
+    state.params.append(handle.flat_param)
+    state._handle = handle
+    state._fully_sharded_module_to_handle[handle._fully_sharded_module] = handle
+    cpu_device = torch.device("cpu")
+    if state.cpu_offload.offload_params and handle.flat_param.device != cpu_device:
+        handle.flat_param_to(cpu_device)
+
+
+def _get_ignored_modules(
+    root_module: nn.Module,
+    _ignored_modules: Optional[Iterable[torch.nn.Module]],
+) -> Set[nn.Module]:
+    """
+    Check that ``_ignored_modules`` is an iterable of ``nn.Module`` s without any FSDP instances.
+
+    Return the modules contained in their module
+    subtrees as a :class:`set`. Nested FSDP instances are excluded, but their
+    already-computed ignored modules are included.
+
+    ``_ignored_modules`` represents the argument passed by the user to FSDP.
+    """
+    msg_prefix = "`ignored_modules` should be an iterable of `torch.nn.Module`s "
+    try:
+        ignored_root_modules = (
+            set(_ignored_modules) if _ignored_modules is not None else set()
+        )
+    except TypeError as e:
+        raise TypeError(msg_prefix + f"but got {type(_ignored_modules)}") from e
+    for module in ignored_root_modules:
+        if not isinstance(module, torch.nn.Module):
+            raise TypeError(msg_prefix + f"but got an iterable with {type(module)}")
+        if _get_module_fsdp_state(module):
+            # TODO: We may relax this by taking the FSDP instance's wrapped
+            # module to provide more flexibility to the user.
+            raise ValueError("`ignored_modules` should not include FSDP modules")
+    # Treat modules that cannot compose with `fully_shard` as ignored modules,
+    # meaning that their subtrees are ignored
+    for module in root_module.modules():
+        if not traversal_utils._composable(module):
+            ignored_root_modules.add(module)
+    # NOTE: Even if `ignored_root_modules` is empty, do not return early so
+    # that this FSDP instance can get any ignored modules from its children.
+
+    # Include child modules and exclude nested FSDP modules themselves
+    ignored_modules = {
+        child
+        for module in ignored_root_modules
+        for child in module.modules()
+        if not isinstance(child, fsdp_file.FullyShardedDataParallel)
+    }
+    if root_module in ignored_modules:
+        warnings.warn(
+            "Trying to ignore the top-level module passed into the FSDP "
+            "constructor itself will result in all parameters being "
+            f"ignored and is not well-supported: {module}"
+        )
+    # Include nested FSDP modules' ignored modules
+    for submodule in root_module.modules():
+        optional_fsdp_state = _get_module_fsdp_state(submodule)
+        if optional_fsdp_state is not None:
+            assert hasattr(optional_fsdp_state, "_ignored_modules")
+            ignored_modules.update(optional_fsdp_state._ignored_modules)
+    return ignored_modules
+
+
+def _get_ignored_params(
+    root_module: torch.nn.Module,
+    ignored_modules: Set[torch.nn.Module],
+    ignored_parameters: Optional[Iterable[torch.nn.Parameter]] = None,
+) -> Set[torch.nn.Parameter]:
+    """
+    Return the parameters of the modules in ``ignored_modules`` and the parameters in ``ignored_parameters``.
+
+    :class:`FlatParameter` s are excluded from the result.
+    """
+    all_ignored_params: Set[torch.nn.Parameter] = set()
+
+    params_in_ignored_modules = {
+        p for m in ignored_modules for p in m.parameters() if not _is_fsdp_flattened(p)
+    }
+
+    all_ignored_params.update(params_in_ignored_modules)
+
+    if ignored_parameters is not None:
+        params_in_ignored_parameters = {
+            p for p in ignored_parameters if not _is_fsdp_flattened(p)
+        }
+        all_ignored_params.update(params_in_ignored_parameters)
+
+    # Always include nested FSDP modules' ignored parameters
+    for submodule in root_module.modules():
+        optional_fsdp_state = _get_module_fsdp_state(submodule)
+        if optional_fsdp_state is not None:
+            assert hasattr(optional_fsdp_state, "_ignored_params")
+            all_ignored_params.update(optional_fsdp_state._ignored_params)
+
+    return all_ignored_params
+
+
+def _get_ignored_buffer_names(
+    root_module: torch.nn.Module,
+    ignored_modules: Set[torch.nn.Module],
+) -> Set[str]:
+    """Return the cleaned buffer FQNs in ``ignored_modules``."""
+    all_ignored_buffer_names: Set[str] = set()
+
+    buffers_in_ignored_modules = {
+        buffer for m in ignored_modules for buffer in m.buffers()
+    }
+
+    all_ignored_buffer_names.update(
+        {
+            clean_tensor_name(buffer_name)
+            for buffer_name, buffer in root_module.named_buffers()
+            if buffer in buffers_in_ignored_modules
+        }
+    )
+
+    # Always include nested FSDP modules' ignored buffer names
+    for submodule in root_module.modules():
+        optional_fsdp_state = _get_module_fsdp_state(submodule)
+        if optional_fsdp_state is not None:
+            assert hasattr(optional_fsdp_state, "_ignored_buffer_names")
+            all_ignored_buffer_names.update(optional_fsdp_state._ignored_buffer_names)
+
+    return all_ignored_buffer_names
+
+
+def _get_buffer_names(root_module: nn.Module) -> Set[str]:
+    """Return the fully prefixed names of all buffers in the module hierarchy rooted at ``root_module`` as a class:`set`."""
+    return {
+        clean_tensor_name(buffer_name) for buffer_name, _ in root_module.named_buffers()
+    }
+
+
+def _check_single_device_module(
+    module: nn.Module,
+    ignored_params: Set[nn.Parameter],
+    device_id: Optional[Union[int, torch.device]],
+) -> None:
+    """
+    Raise an error if ``module`` has original parameters on multiple devices, ignoring the parameters in ``ignored_params``.
+
+    Thus, after this method, the
+    module must be either fully on the CPU or fully on a non-CPU device.
+    """
+    devices = {param.device for param in _get_orig_params(module, ignored_params)}
+    # We allow module to be partially on CPU and partially on GPU if device_id is not
+    # None, since the device_id arg will result in the CPU portion being moved to
+    # GPU. This is useful in cases where part of the module may be parallelized
+    # by another algorithm and may already be on GPU. We'd like to enforce device_id
+    # to not be None, otherwise we'd flatten parameters in a mixed module which is
+    # not supported.
+    if len(devices) == 2 and torch.device("cpu") in devices:
+        if device_id is None:
+            raise RuntimeError(
+                "To support a module with both CPU and GPU params, "
+                "please pass in device_id argument."
+            )
+    elif len(devices) > 1:
+        raise RuntimeError(
+            f"FSDP only supports single device modules but got params on {devices}"
+        )
+
+
+def _get_device_from_device_id(
+    device_id: Optional[Union[int, torch.device]],
+    rank: int,
+    device_handle: _FSDPDeviceHandle,
+) -> Optional[torch.device]:
+    """
+    Return a ``torch.device`` for the specified ``device_id``.
+
+    Processes ``device_id`` and returns either the corresponding device or
+    ``None`` if ``device_id`` is ``None``.
+    """
+    if device_id is None:
+        return None
+    device = (
+        device_id if isinstance(device_id, torch.device) else torch.device(device_id)
+    )
+    if device.type != "cpu" and device.index is None:
+        warnings.warn(
+            f"FSDP got the argument `device_id` {device_id} on rank "
+            f"{rank}, which does not have an explicit index. "
+            f"FSDP will use the current device {device_handle.current_device()}. "
+            f"If this is incorrect, please explicitly call `torch.{device.type}.set_device()` "
+            "before FSDP initialization or pass in the explicit device "
+            "index as the `device_id` argument."
+        )
+        device = torch.device(device_handle.current_device())
+    return device
+
+
+def _need_to_materialize_module(
+    module: nn.Module,
+    ignored_params: Set[nn.Parameter],
+    ignored_modules: Set[nn.Module],
+) -> Tuple[bool, bool]:
+    """
+    Return if ``module`` has parameters on meta device and if ``module`` is using torchdistX deferred initialization.
+
+    At most of the returned bools can
+    be ``True``. If either is ``True``, then ``module`` needs to be
+    materialized.
+    """
+    managed_params = list(_get_orig_params(module, ignored_params))
+    is_meta_module = any(param.is_meta for param in managed_params)
+    # TODO: We need to establish a contract for FSDP and buffers. For now, we
+    # skip checking for meta buffers from ignored modules. We should consider
+    # refactoring the initialization holistically to avoid so many traversals.
+    for submodule in module.modules():
+        if submodule in ignored_modules:
+            continue
+        for buf in submodule.buffers(recurse=False):
+            is_meta_module |= buf.is_meta
+    is_torchdistX_deferred_init = (
+        not is_meta_module
+        and _TORCHDISTX_AVAIL
+        and any(fake.is_fake(param) for param in managed_params)
+    )
+    return is_meta_module, is_torchdistX_deferred_init
+
+
+def _materialize_with_param_init_fn(
+    root_module: nn.Module,
+    param_init_fn: Callable[[nn.Module], None],
+    ignored_modules: Set[nn.Module],
+) -> None:
+    if not callable(param_init_fn):
+        raise ValueError(
+            f"Expected {param_init_fn} to be callable but got {type(param_init_fn)}"
+        )
+    modules_to_materialize = _get_modules_to_materialize(root_module, ignored_modules)
+    for module in modules_to_materialize:
+        param_init_fn(module)
+
+
+def _materialize_meta_module(
+    root_module: nn.Module,
+    device_from_device_id: Optional[torch.device],
+    ignored_modules: Set[nn.Module],
+    device_handle: _FSDPDeviceHandle,
+):
+    # Run default meta device initialization
+    materialization_device = device_from_device_id or torch.device(
+        device_handle.current_device()
+    )
+    modules_to_materialize = _get_modules_to_materialize(root_module, ignored_modules)
+    module = None
+    try:
+        # Assume that each module's `reset_parameters()` only initializes its
+        # own parameters and not those of its children
+        with torch.no_grad():
+            for module in modules_to_materialize:
+                # As a contract to the user, only call `reset_parameters()` if
+                # the module has directly managed parameters/buffers
+                module_state_iter = itertools.chain(
+                    module.parameters(recurse=False), module.buffers(recurse=False)
+                )
+                has_module_states = len(list(module_state_iter)) > 0
+                if has_module_states:
+                    module.to_empty(device=materialization_device, recurse=False)
+                    module.reset_parameters()  # type: ignore[operator]
+    except BaseException as e:
+        warnings.warn(
+            "Unable to call `reset_parameters()` for module on meta "
+            f"device with error {str(e)}. Please ensure that your module of"
+            f"type {type(module)} implements a `reset_parameters()` method."  # type: ignore[possibly-undefined]
+        )
+        raise e
+
+
+def _get_modules_to_materialize(
+    root_module: nn.Module, ignored_modules: Set[nn.Module]
+) -> List[nn.Module]:
+    # Run BFS to collect the modules to materialize via `reset_parameters()`,
+    # stopping at any module with FSDP already applied or at ignored modules.
+    modules_to_materialize: List[nn.Module] = []
+    queue = collections.deque([root_module])
+    visited_modules: Set[nn.Module] = {root_module}
+    while queue:
+        module = queue.popleft()
+        modules_to_materialize.append(module)
+        for child_module in module.children():
+            if (
+                child_module not in visited_modules
+                and _get_module_fsdp_state(child_module) is None
+                and child_module not in ignored_modules
+            ):
+                visited_modules.add(child_module)
+                queue.append(child_module)
+    return modules_to_materialize
+
+
+def _move_module_to_device(
+    module: nn.Module,
+    ignored_params: Set[nn.Parameter],
+    ignored_buffers: Set[torch.Tensor],
+    device_from_device_id: Optional[torch.device],
+) -> None:
+    """
+    Move ``module`` depending on ``device_from_device_id`` and its current device.
+
+    This includes moving ignored modules' parameters.
+
+    - If ``device_from_device_id`` is not ``None``, then this moves
+    ``module`` to the device.
+    - If ``device_from_device_id`` is ``None``, then this does not move
+    ``module`` but warns the user if it is on CPU.
+
+    Precondition: ``_check_single_device_module()``.
+    """
+    cpu_device = torch.device("cpu")
+    if device_from_device_id is not None:
+        # BFS from `module` without traversing any nested FSDP instances to
+        # collect the parameters/buffers that have not yet been managed
+        queue: Deque[nn.Module] = collections.deque()
+        queue.append(module)
+        params: List[nn.Parameter] = []
+        buffers: List[torch.Tensor] = []
+        while queue:
+            curr_module = queue.popleft()
+            # NOTE: We include a check to only move parameters/buffers that are
+            # on CPU device. If they are on a CUDA device different from the
+            # one specified by `device_id`, then this does NOT move them. This
+            # is so that we can raise an error in `_get_compute_device()`.
+            params.extend(
+                param
+                for param in curr_module.parameters(recurse=False)
+                if param.device == cpu_device
+            )
+            buffers.extend(
+                buffer
+                for buffer in curr_module.buffers(recurse=False)
+                if buffer.device == cpu_device
+            )
+            for submodule in curr_module.children():
+                if not isinstance(submodule, fsdp_file.FullyShardedDataParallel):
+                    queue.append(submodule)
+        params_to_move = [p for p in params if p not in ignored_params]
+        bufs_to_move = [p for p in buffers if p not in ignored_buffers]
+        _move_states_to_device(params_to_move, bufs_to_move, device_from_device_id)
+        return
+    param = next(_get_orig_params(module, ignored_params), None)
+    if param is not None and param.device == cpu_device:
+        _warn_cpu_init()
+
+
+def _move_states_to_device(
+    params: List[nn.Parameter],
+    buffers: List[torch.Tensor],
+    device_from_device_id: Optional[torch.device],
+) -> None:
+    """
+    Move states to the specified device.
+
+    Precondition: ``_check_single_device_module()`` and module's parameters and
+    buffers have been materialized if needed.
+    """
+    if len(params) == 0 and len(buffers) == 0:
+        return
+    if len(params) > 0:
+        current_device = params[0].device
+    elif len(buffers) > 0:
+        current_device = buffers[0].device
+    cpu_device = torch.device("cpu")
+    if device_from_device_id is not None:
+        # Move the parameters and buffers like the `.data` code path in
+        # `nn.Module._apply()`, which underlies `nn.Module.to()`
+        for param in params:
+            with torch.no_grad():
+                param.data = param.to(device_from_device_id)
+                if param.grad is not None:
+                    param.grad.data = param.grad.to(device_from_device_id)
+        for buffer in buffers:
+            buffer.data = buffer.to(device_from_device_id)
+    elif current_device == cpu_device:  # type: ignore[possibly-undefined]
+        _warn_cpu_init()
+
+
+def _warn_cpu_init():
+    warnings.warn(
+        "The passed-in `module` is on CPU and will thus have FSDP's sharding "
+        "initialization run on CPU, which may be slower than on GPU. We "
+        "recommend passing in the `device_id` argument for FSDP to move "
+        "`module` to GPU for the sharding initialization. `module` must also "
+        "be on GPU device to work with the `sync_module_states=True` flag "
+        "since that requires GPU communication."
+    )
+
+
+def _get_compute_device(
+    module: nn.Module,
+    ignored_params: Set[nn.Parameter],
+    device_from_device_id: Optional[torch.device],
+    rank: int,
+    device_handle: _FSDPDeviceHandle,
+) -> torch.device:
+    """
+    Determine and return this FSDP instance's compute device.
+
+    If the module is already on a non-CPU device, then the compute device is that non-CPU
+    device. If the module is on CPU, then the compute device is the current
+    device.
+
+    Since this method should be called after materializing the module, any
+    non-CPU device should not be meta device. For now, the compute device is
+    always a CUDA or CUDA-like device with its explicit index.
+
+    Precondition: ``_check_single_device_module()`` and
+    ``_move_module_to_device()``.
+    """
+    param = next(_get_orig_params(module, ignored_params), None)
+    if param is not None and param.device.type != "cpu":
+        compute_device = param.device  # Determined by model param placement
+    else:
+        compute_device = torch.device(device_handle.current_device())
+    if device_from_device_id is not None and compute_device != device_from_device_id:
+        raise ValueError(
+            f"Inconsistent compute device and `device_id` on rank {rank}: "
+            f"{compute_device} vs {device_from_device_id}"
+        )
+    return compute_device
+
+
+# TODO: See how to deprecate!
+def _sync_module_params_and_buffers(
+    module: nn.Module,
+    params: List[nn.Parameter],
+    process_group: dist.ProcessGroup,
+) -> None:
+    """
+    Synchronize module states (i.e. parameters ``params`` and all not-yet-synced buffers) by broadcasting from rank 0 to all ranks.
+
+    Precondition: ``sync_module_states == True`` and ``self.process_group`` has
+    been set.
+    """
+    module_states: List[torch.Tensor] = []
+    for buffer in module.buffers():
+        # Avoid re-synchronizing buffers in case of nested wrapping
+        if not getattr(buffer, FSDP_SYNCED, False):
+            setattr(buffer, FSDP_SYNCED, True)
+            detached_buffer = buffer.detach()
+            if is_traceable_wrapper_subclass(detached_buffer):
+                # NOTE: Here we assume no nested subclasses, at most one level of subclass
+                # in both model's buffers and params
+                attrs, _ = detached_buffer.__tensor_flatten__()  # type: ignore[attr-defined]
+                inner_buffers = [getattr(detached_buffer, attr) for attr in attrs]
+                module_states.extend(inner_buffers)
+            else:
+                module_states.append(detached_buffer)
+
+    for param in params:
+        detached_param = param.detach()
+        if is_traceable_wrapper_subclass(detached_param):
+            attrs, _ = detached_param.__tensor_flatten__()  # type: ignore[attr-defined]
+            inner_params = [getattr(detached_param, attr) for attr in attrs]
+            module_states.extend(inner_params)
+        else:
+            module_states.append(detached_param)
+
+    _check_module_states_for_sync_module_states(module_states)
+    _sync_params_and_buffers(
+        process_group,
+        module_states,
+        PARAM_BROADCAST_BUCKET_SIZE,
+        src=0,
+    )
+
+
+def _check_module_states_for_sync_module_states(
+    module_states: List[torch.Tensor],
+) -> None:
+    if module_states and any(
+        tensor.device == torch.device("cpu") for tensor in module_states
+    ):
+        raise ValueError(
+            "The module has CPU parameters or buffers when `sync_module_states=True`, "
+            "which requires them to be on GPU. Please specify the `device_id` argument "
+            "or move the module to GPU before passing it to FSDP."
+        )
+
+
+def _get_orig_params(
+    module: nn.Module,
+    ignored_params: Set[nn.Parameter],
+) -> Iterator[nn.Parameter]:
+    """
+    Return an iterator over the original parameters in ``module``.
+
+    The iterator does not return
+    the parameters in ``ignored_params``, any ``FlatParameter`` s (which may be
+    present due to nested FSDP wrapping), or any original parameters already
+    flattened (only relevant when ``use_orig_params=True``).
+    """
+    param_gen = module.parameters()
+    try:
+        while True:
+            param = next(param_gen)
+            if param not in ignored_params and not _is_fsdp_flattened(param):
+                yield param
+    except StopIteration:
+        pass
+
+
+def _check_orig_params_flattened(
+    fsdp_module,
+    ignored_params: Set[nn.Parameter],
+) -> None:
+    """
+    Check that original parameters in ``fsdp_module`` have been flattened.
+
+    The flattened parameters are made
+    invisible to ``named_parameters()`` for the module hierarchy rooted at
+    ``fsdp_module``. This should be called as a sanity check after flattening
+    the wrapped module's parameters.
+    """
+    for param_name, param in _named_parameters_with_duplicates(fsdp_module):
+        if param not in ignored_params and not _is_fsdp_flattened(param):
+            raise RuntimeError(
+                f"Found an unflattened parameter: {param_name}; "
+                f"{param.size()} {param.__class__}"
+            )
+
+
+def _get_default_comm_hook(sharding_strategy: ShardingStrategy):
+    return (
+        default_hooks.allreduce_hook
+        if sharding_strategy == ShardingStrategy.NO_SHARD
+        else default_hooks.reduce_scatter_hook
+    )
+
+
+def _get_default_comm_hook_state(
+    process_group: dist.ProcessGroup,
+) -> default_hooks.DefaultState:
+    return default_hooks.DefaultState(process_group=process_group)

janus/lib/python3.10/site-packages/torch/distributed/fsdp/_optim_utils.py

@@ -0,0 +1,2091 @@
+# mypy: allow-untyped-defs
+import copy
+import functools
+import logging
+import warnings
+from contextlib import ExitStack
+from dataclasses import dataclass, field
+from typing import (
+    Any,
+    cast,
+    Dict,
+    Iterable,
+    Iterator,
+    List,
+    NamedTuple,
+    no_type_check,
+    Optional,
+    Sequence,
+    Set,
+    Tuple,
+    TYPE_CHECKING,
+    Union,
+)
+
+import torch
+import torch.distributed as dist
+import torch.distributed.fsdp._traversal_utils as traversal_utils
+import torch.nn as nn
+from torch.distributed._state_dict_utils import _gather_state_dict
+from torch.distributed.distributed_c10d import _get_pg_default_device
+from torch.distributed.fsdp._common_utils import (
+    _apply_to_modules,
+    _FSDPState,
+    _get_module_fsdp_state_if_fully_sharded_module,
+    _get_param_to_fqns,
+    _module_handle,
+    _named_parameters_with_duplicates,
+    clean_tensor_name,
+)
+from torch.distributed.fsdp._debug_utils import SimpleProfiler
+from torch.distributed.fsdp._flat_param import FlatParameter, FlatParamHandle
+from torch.distributed.fsdp._fsdp_extensions import (
+    _ext_chunk_dtensor,
+    _ext_chunk_tensor,
+)
+from torch.distributed.fsdp._runtime_utils import (
+    _lazy_init,
+    _reset_flat_param_grad_info_if_needed,
+)
+from torch.distributed.fsdp.api import (
+    ShardingStrategy,
+    StateDictSettings,
+    StateDictType,
+)
+from torch.distributed.tensor import DTensor, Replicate
+from torch.utils._pytree import tree_map_only
+
+
+if TYPE_CHECKING:
+    from torch.distributed._shard.sharded_tensor import ShardedTensor
+
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class FSDPParamInfo:
+    state: _FSDPState
+    handle: FlatParamHandle
+    param_indices: Dict[str, int]
+    param_requires_grad: List[bool]
+
+
+def sorted_items(dictionary: Dict[str, Any]) -> Iterator[Tuple[str, Any]]:
+    keys = sorted(dictionary.keys())
+    for k in keys:
+        yield k, dictionary[k]
+
+
+@dataclass
+class _ConsolidatedOptimState:
+    """
+    This holds the consolidated optimizer state on the target rank. Positive-
+    dimension tensor state is communicated across ranks, while zero-dimension
+    tensor state and non-tensor state is taken directly from the target rank.
+
+    PyTorch version 1.12 moved to using zero-dimension tensors for scalar
+    values, but user implemented optimizers may still use float (i.e. a
+    non-tensor). Thus, we support both and handle them identically.
+
+    Attributes:
+        tensor_state (Dict[str, torch.Tensor]): Mapping from positive-dimension
+            tensor state name to the unsharded flat tensor representing the
+            state.
+        zero_dim_tensor_state (Dict[str, torch.Tensor]): Mapping from zero-
+            dimension tensor state name to its value.
+        non_tensor_state (Dict[str, Any]): Mapping from non-tensor state
+            name to its value.
+    """
+
+    tensor_state: Dict[str, torch.Tensor] = field(default_factory=dict)
+    zero_dim_tensor_state: Dict[str, torch.Tensor] = field(default_factory=dict)
+    non_tensor_state: Dict[str, Any] = field(default_factory=dict)
+
+
+class _PosDimTensorInfo(NamedTuple):
+    """
+    Meatadata for positive-dimension tensors used internally for
+    :meth:`scatter_full_optim_state_dict`.
+
+    Attributes:
+        shape (torch.Size): Sharded tensor shape (which is equal to the
+            unsharded tensor shape if the tensor is optimizer state for a
+            non-FSDP parameter and is hence not sharded).
+        dtype (torch.dtype): Data type of the tensor.
+    """
+
+    shape: torch.Size
+    dtype: torch.dtype
+
+
+class _OptimStateKey(NamedTuple):
+    """
+    This represents an optimizer state key that may be used commonly across
+    ranks. It is based on the unflattened parameter names rather than parameter
+    IDs to make it independent of each rank's own optimizer construction.
+    """
+
+    unflat_param_names: Tuple[str, ...]
+    is_fsdp_managed: bool
+
+
+def _unflatten_optim_state(
+    fsdp_param_info: FSDPParamInfo,
+    flat_param_state: Dict[str, Any],
+    to_save: bool,
+    shard_state: bool,
+    cpu_offload: bool,
+) -> List[Dict[str, Any]]:
+    """
+    Unflattens the optimizer state, consisting of the "state" part and the
+    "param_groups" part. Unflattening the "state" part involves consolidating
+    the state on the target rank and remapping from flattened to unflattened
+    parameter IDs, and the "param_groups" part only involves remapping from
+    flattened to unflattened parameter IDs.
+
+    Args:
+        fsdp_param_info (FSDPParamInfo): The FSDP state, the handle, and a
+            mapping from FQN to original parameter index.
+        flat_param_state (Dict[str, Any]): Entry for the flat parameter in the
+            "state" part of the optimizer state dict.
+        to_save (bool): Whether to save the state on this rank.
+
+    Returns:
+        List[Dict[str, Any]]: A :class:`list` holding the entries in the
+        "state" part of the optimizer state dict corresponding to the
+        unflattened parameters comprising the flat parameter if on the target
+        rank or an empty :class:`list` otherwise. The final optimizer state
+        dict will need to map these entries using the proper unflattened
+        parameter IDs.
+    """
+    assert (
+        not shard_state or to_save
+    ), "If ``shard_state`` is True, ``to_save`` has to be True."
+    consolidated_state = _communicate_optim_state(
+        fsdp_param_info,
+        flat_param_state,
+    )
+    if to_save:
+        unflat_param_state = _unflatten_communicated_optim_state(
+            fsdp_param_info,
+            consolidated_state,
+            shard_state,
+        )
+        for optim_state in unflat_param_state:
+            # We can't use .items() below cuz we'd run into a concurrent modification error
+            if cpu_offload:
+                for key in list(optim_state.keys()):
+                    state = optim_state[key]
+                    if not isinstance(state, torch.Tensor):
+                        continue
+                    optim_state[key] = state.cpu()
+        return unflat_param_state
+    else:
+        return []
+
+
+def _is_zero_dim_tensor(x: Any) -> bool:
+    return torch.is_tensor(x) and x.dim() == 0
+
+
+def _communicate_optim_state(
+    fsdp_param_info: FSDPParamInfo,
+    flat_param_state: Dict[str, Any],
+) -> _ConsolidatedOptimState:
+    """
+    Communicates the optimizer state for a flat parameter across ranks. All
+    ranks will hold the entire non-sharded optimizer state on GPU.
+
+    If ``N`` is the number of tensor optimizer states in the optimizer state
+    dict, then the communication complexity is 0 if ``N = 0`` and ``N + 1``
+    otherwise (where the plus 1 comes from all-gathering the padding per rank).
+
+    Args:
+        fsdp_param_info (FSDPParamInfo): The FSDP state, the handle, and a
+            mapping from FQN to original parameter index.
+        flat_param_state (Dict[str, Any]): The entry in the "state" part of the
+            optimizer state dict corresponding to the flat parameter.
+
+    Returns:
+        ConsolidatedOptimState: Consolidated optimizer state for the target
+        flat parameter.
+    """
+    fsdp_state = fsdp_param_info.state
+    flat_param = fsdp_param_info.handle.flat_param
+    state = _ConsolidatedOptimState()
+    tensor_state, zero_dim_tensor_state, non_tensor_state = (
+        state.tensor_state,
+        state.zero_dim_tensor_state,
+        state.non_tensor_state,
+    )
+
+    for state_name, value in sorted_items(flat_param_state):
+        # Positive-dimension tensor state: communicate across ranks
+        if torch.is_tensor(value) and value.dim() > 0:
+            # If the parameter is not sharded, then neither is the
+            # positive-dimension tensor state, so no need to communicate it --
+            # we take the target rank's value
+            if (
+                fsdp_state.world_size == 1
+                or fsdp_state.sharding_strategy == ShardingStrategy.NO_SHARD
+            ):
+                tensor_state[state_name] = value
+                continue
+            assert (
+                fsdp_state.compute_device is not None
+            ), "compute_device has not been initialized"
+            if value.device.type != fsdp_state.compute_device.type:
+                value = value.to(fsdp_state.compute_device)
+            # Assume that positive-dimension tensor optimizer state
+            # has the same shape as the sharded flat parameter
+            buffer_size = flat_param._full_param_padded.size()  # type: ignore[attr-defined]
+            tensor_buffer = value.new_zeros(*buffer_size)
+            dist.all_gather_into_tensor(
+                tensor_buffer, value, group=fsdp_state.process_group
+            )
+            fsdp_state._device_handle.synchronize()
+            unpadded_numel = cast(
+                nn.Parameter, flat_param._unpadded_unsharded_size
+            ).numel()
+            tensor_state[state_name] = tensor_buffer[:unpadded_numel]
+        # Zero-dimension tensor state and non-tensor state: take this rank's
+        # value directly
+        else:
+            if _is_zero_dim_tensor(value):
+                zero_dim_tensor_state[state_name] = value.detach().clone()
+            else:
+                non_tensor_state[state_name] = value
+    return state
+
+
+def _unflatten_communicated_optim_state(
+    fsdp_param_info: FSDPParamInfo,
+    state: _ConsolidatedOptimState,
+    shard_state: bool,
+) -> List[Dict[str, Any]]:
+    """
+    Unflattens the communicated optimizer state (given by ``tensor_state``,
+    ``non_tensor_state``, and ``zero_dim_tensor_state``) for a single flat
+    parameter. This should only be called on the target rank.
+
+    Args:
+        fsdp_param_info (FSDPParamInfo): The FSDP state, the handle, and a
+            mapping from FQN to original parameter index.
+        state (_ConsolidatedOptimState): Consolidated optimizer state.
+
+    Returns:
+        List[Dict[str, Any]]: A :class:`list` holding the entries in the
+        "state" part of the optimizer state dict corresponding to the
+        unflattened parameters comprising the flat parameter. The final
+        optimizer state dict will need to map these entries using the proper
+        unflattened parameter IDs.
+    """
+    fsdp_state = fsdp_param_info.state
+    handle = fsdp_param_info.handle
+    flat_param = handle.flat_param
+    unflat_param_state: List[Dict[str, Any]] = []
+    flat_param_views: Dict[str, Iterator] = {}
+    num_unflat_params = flat_param._num_params
+    tensor_state, zero_dim_tensor_state, non_tensor_state = (
+        state.tensor_state,
+        state.zero_dim_tensor_state,
+        state.non_tensor_state,
+    )
+
+    for _ in range(num_unflat_params):
+        unflat_state_param = {}
+        # Add positive-dimension tensor state: unflatten with views
+        for state_name, flat_tensor in sorted_items(tensor_state):
+            views_generated = state_name in flat_param_views
+            if not views_generated:
+                views = handle._get_unflat_views(flat_tensor)
+                flat_param_views[state_name] = views
+            else:
+                views = flat_param_views[state_name]
+            optim_state: Union[torch.Tensor, ShardedTensor, DTensor] = next(views)
+            if shard_state:
+                osd_config = fsdp_state._optim_state_dict_config
+                if getattr(osd_config, "_use_dtensor", False):
+                    assert fsdp_state._device_mesh is not None
+                    optim_state = _ext_chunk_dtensor(
+                        optim_state,
+                        fsdp_state.rank,
+                        fsdp_state._device_mesh,
+                        fsdp_state._fsdp_extension,
+                    )
+                else:
+                    assert fsdp_state.process_group is not None
+                    optim_state = _ext_chunk_tensor(
+                        optim_state,
+                        fsdp_state.rank,
+                        fsdp_state.world_size,
+                        fsdp_state._device_handle.device_count(),
+                        fsdp_state.process_group,
+                        fsdp_state._fsdp_extension,
+                    )
+            unflat_state_param[state_name] = optim_state
+
+        # Add zero-dimension tensor state: take the target rank's value
+        for state_name, zero_dim_tensor in sorted_items(zero_dim_tensor_state):
+            unflat_state_param[state_name] = zero_dim_tensor
+        # Add non-tensor state: take the target rank's value
+        for state_name, non_tensor in sorted_items(non_tensor_state):
+            unflat_state_param[state_name] = non_tensor
+        unflat_param_state.append(unflat_state_param)
+    return unflat_param_state
+
+
+def _broadcast_processed_state(
+    fsdp_state: _FSDPState,
+    optim_state: Dict[str, Any],
+    group: Optional[dist.ProcessGroup],
+) -> Dict[str, Any]:
+    objects: List[Any] = [None]
+    if dist.get_rank(group) == 0:
+        objects[0] = tree_map_only(
+            torch.Tensor,
+            lambda v: v.cpu() if v.dim() == 0 else _PosDimTensorInfo(v.shape, v.dtype),  # type: ignore[union-attr]
+            optim_state,
+        )
+    dist.broadcast_object_list(objects, src=0, group=group)
+    if dist.get_rank(group) == 0:
+        return optim_state
+    else:
+        return objects[0]
+
+
+def _broadcast_state(
+    fsdp_state: _FSDPState, state: Any, group: Optional[dist.ProcessGroup]
+) -> Any:
+    if dist.get_rank(group) == 0:
+        if not isinstance(state, torch.Tensor) or state.dim() == 0:
+            return state
+        tensor = state.to(fsdp_state.compute_device)
+    else:
+        if isinstance(state, torch.Tensor):
+            assert state.dim() == 0, (
+                "For non-zero ranks, a tensor state should have zero dimension, "
+                "but got the state with shape {state.shape()}."
+            )
+            return state
+        elif not isinstance(state, _PosDimTensorInfo):
+            return state
+        tensor = torch.zeros(
+            state.shape, dtype=state.dtype, device=fsdp_state.compute_device
+        )
+    dist.broadcast(tensor, src=0, group=group)
+    return tensor
+
+
+def _shard_orig_param_state(
+    fsdp_param_info: FSDPParamInfo,
+    fqn: str,
+    optim_state: Dict[str, Any],
+) -> Dict[str, Any]:
+    """
+    Shard the optimizer state for the original parameter with the name ``fqn``.
+    This API should only be used when ``use_orig_params`` is True.
+    """
+    if not optim_state:
+        return {}
+    fsdp_state = fsdp_param_info.state
+    flat_param = fsdp_param_info.handle.flat_param
+    param_idx = fsdp_param_info.param_indices[fqn]
+    shard_param_info = flat_param._shard_param_infos[param_idx]  # type: ignore[attr-defined]
+    optim_state = _gather_state_dict(
+        optim_state, pg=fsdp_state.process_group, device=fsdp_state.compute_device
+    )
+    if not shard_param_info.in_shard:
+        return {}
+    # Flatten and shard the state.
+    new_optim_state: Dict[str, Any] = {}
+    intra_param_start_idx = shard_param_info.intra_param_start_idx
+    intra_param_end_idx = shard_param_info.intra_param_end_idx
+    for state_name, value in optim_state.items():
+        if (
+            torch.is_tensor(value)
+            and value.dim() > 0
+            and fsdp_state.sharding_strategy != ShardingStrategy.NO_SHARD
+        ):
+            value = value.flatten()[intra_param_start_idx : intra_param_end_idx + 1].clone()  # type: ignore[operator]
+        new_optim_state[state_name] = value
+    return new_optim_state
+
+
+def _flatten_optim_state_dict(
+    optim_state_dict: Dict[str, Any],
+    model: nn.Module,
+    use_orig_params: bool = False,
+    optim: Optional[torch.optim.Optimizer] = None,
+    rank0_only: bool = False,
+    group: Optional[dist.ProcessGroup] = None,
+) -> Dict[str, Any]:
+    """
+    Flattens the full optimizer state dict, still keying by unflattened parameter
+    names.
+
+    If ``use_orig_params`` is True, each rank will have all FSDP-managed
+    parameters but some of these parameters may be empty due to the sharding.
+    For a regular optim.Optimizer, states for those empty parameters will
+    not be initialized. So, when aggregating the FQNs across ranks, no assert
+    will be raised on a rank even if it does not have all the states -- it is
+    valid and FSDP know how to aggregate them. However, FSDP has to ignore
+    handling those parameters that are not managed by FSDP and do not exist on
+    the local rank -- it is managed by other parallelism and FSDP does not
+    know ho to handle/aggregate them.
+
+    Note that ``_flatten_tensor_optim_state`` does not need ``optim`` to
+    flatten/shard the state. However, NamedOptimizer and KeyedOptimizer require
+    all the states even if the corresponding parameters are empty. To this end,
+    ``optim`` will be used to to get the initial state of the empty parameters.
+    ``optim`` should only be non-None if the ``optim` is KeyedOptimizer or
+    NamedOptimizer.
+
+    Returns:
+        Dict[str, Any]: The flattened optimizer state dict.
+    """
+    SimpleProfiler.reset()
+
+    unflat_osd = optim_state_dict
+    if "state" not in unflat_osd and not rank0_only:
+        raise ValueError(
+            '`optim_state_dict` must have the keys "state"'
+            "to be a valid optimizer state dict"
+        )
+    param_to_fqns = _get_param_to_fqns(model)
+    fqn_to_fsdp_param_info = _get_fqn_to_fsdp_param_info(model)
+    fsdp_state = next(iter(fqn_to_fsdp_param_info.values())).state
+
+    # Broadcast unflat_osd without non-scalar tensor if rank0_only is True.
+    if rank0_only:
+        unflat_osd = _broadcast_processed_state(fsdp_state, unflat_osd, group=group)
+
+    # Construct the "state" part
+    flat_osd_state: Dict[Union[_OptimStateKey, str], Any] = {}
+    unflat_osd_state = unflat_osd["state"]
+    all_state_keys = set(unflat_osd_state.keys())
+
+    for param, fqns in param_to_fqns.items():
+        fqn = fqns[0]
+        if fqn not in unflat_osd_state:
+            continue
+        all_state_keys.difference_update(fqns)
+
+        if rank0_only:
+            for fqn in fqns:
+                if not unflat_osd_state[fqn]:
+                    continue
+                for state_name in unflat_osd_state[fqn].keys():
+                    unflat_osd_state[fqn][state_name] = _broadcast_state(
+                        fsdp_state, unflat_osd_state[fqn][state_name], group=group
+                    )
+            fqn = fqns[0]
+        if fqn in fqn_to_fsdp_param_info:
+            fsdp_param_info = fqn_to_fsdp_param_info[fqn]
+            if use_orig_params:
+                with SimpleProfiler.profile(SimpleProfiler.Type.RESHARDING):
+                    flat_state = _shard_orig_param_state(
+                        fsdp_param_info,
+                        fqn,
+                        unflat_osd_state[fqn],
+                    )
+            else:
+                flat_state = _flatten_optim_state(
+                    fsdp_param_info,
+                    unflat_osd_state,
+                    fqns,
+                )
+            key = _OptimStateKey(tuple(fqns), True)
+            # Only include non-empty states since as expected by
+            # `torch.optim.Optimizer` s unless the optimizer is KeyedOptimizer
+            # or NamedOptimizer.
+            if flat_state:
+                flat_osd_state[key] = flat_state
+            elif use_orig_params:
+                assert (
+                    len(fqns) == 1
+                ), f"use_orig_params is True but there are multiple FQNs, {fqns}."
+                if optim is not None:  # NamedOptimizer or KeyedOptimizer case.
+                    state = optim.state.get(param, None)  # type: ignore[call-overload]
+                    if state is not None:
+                        flat_osd_state[key] = copy.deepcopy(state)
+                    else:
+                        warnings.warn(
+                            f"optim_state[{key}] is not on rank{fsdp_state.rank}."
+                        )
+
+            else:
+                raise RuntimeError(
+                    f"The state of {key} is empty. This should happen when "
+                    "use_orig_params=True."
+                )
+        else:  # do not flatten non-FSDP parameters' states
+            assert len(fqns) == 1
+            key = _OptimStateKey(tuple(fqns), False)
+            flat_osd_state[key] = copy.copy(unflat_osd_state[fqn])
+
+        if rank0_only:
+            for fqn in fqns:
+                if not unflat_osd_state[fqn]:
+                    continue
+                for state_name, param_state in list(unflat_osd_state[fqn].items()):
+                    if fsdp_state.rank > 0:
+                        # Deference the tensor so that PyTorch can collect the memory.
+                        del unflat_osd_state[fqn][state_name]
+                    else:
+                        # Move the tensor in the original osd back to CPU to make the
+                        # original osd unaffected.
+                        unflat_osd_state[fqn][state_name] = unflat_osd_state[fqn][
+                            state_name
+                        ].cpu()
+
+    # Handle user-defined state, states that are not associated with parameters.
+    for key in all_state_keys:
+        user_state = unflat_osd_state[key]
+        if isinstance(user_state, torch.Tensor) and rank0_only and use_orig_params:
+            user_state = _broadcast_state(fsdp_state, user_state, group=group)
+        flat_osd_state[key] = copy.copy(user_state)
+
+    SimpleProfiler.dump_and_reset("FSDP _flatten_optim_state_dict() profiling: ")
+    # Construct the "param_groups" part -- copy as is since it will be
+    # rekeyed later according to the target rank's optimizer
+    # Only copy param_groups if it exists in unflat_osd
+    if "param_groups" in unflat_osd:
+        flat_osd_param_groups = copy.deepcopy(unflat_osd["param_groups"])
+        return {"state": flat_osd_state, "param_groups": flat_osd_param_groups}
+    else:
+        return {"state": flat_osd_state}
+
+
+def _flatten_optim_state(
+    fsdp_param_info: FSDPParamInfo,
+    unflat_osd_state: Dict[str, Dict[str, Any]],
+    unflat_param_names: List[str],
+) -> Dict[str, Any]:
+    """
+    Flattens the optimizer state in ``full_optim_state_dict`` for a single
+    flat parameter in ``fsdp_param_info`` corresponding to the unflattened
+    parameter names in ``unflat_param_names``.
+
+    Args:
+        fsdp_param_info (FSDPParamInfo): The FSDP state, the handle, and a
+            mapping from FQN to original parameter index.
+        unflat_osd_state (Dict[str, Dict[str, Any]]): The "state" part of the
+            optimizer state dict corresponding to the unflattened parameters.
+        unflat_param_names (List[str]): A :class:`list` of unflattened
+            parameter names corresponding to the flat parameter ``flat_param``.
+
+    Returns:
+        Dict[str, Any]: A :class:`dict` mapping state names to their values for
+        a particular flat parameter. The sharded optimizer state dict's "state"
+        part will map a key to this returned value.
+    """
+    fsdp_state = fsdp_param_info.state
+    handle = fsdp_param_info.handle
+    flat_param = handle.flat_param
+    num_unflat_params = len(unflat_param_names)
+    assert num_unflat_params > 0, (
+        "Expects at least one unflattened parameter corresponding to the "
+        "flat parameter"
+    )
+    unflat_param_shapes = flat_param._shapes
+    num_unflat_param_shapes = len(unflat_param_shapes)
+    assert (
+        num_unflat_params == num_unflat_param_shapes
+    ), f"Expects {num_unflat_params} shapes but got {num_unflat_param_shapes}"
+
+    # Check if these unflattened parameters have any optimizer state
+    has_state = [
+        bool(unflat_param_name in unflat_osd_state)
+        for unflat_param_name in unflat_param_names
+    ]
+    # If none of the unflattened parameters comprising this flat parameter have
+    # any state, then we do not want an entry in the optimizer state dict
+    if not any(has_state):
+        return {}  # no need to flatten any state
+    # There may still be some unflattened parameters with state and some
+    # without
+    unflat_param_states = [
+        _gather_state_dict(
+            unflat_osd_state[unflat_param_name],
+            pg=fsdp_state.process_group,
+            device=fsdp_state.compute_device,
+        )
+        if unflat_param_name in unflat_osd_state
+        else None
+        for unflat_param_name in unflat_param_names
+    ]
+    # Check that the unflattened parameters have the same state names
+    state_names = None
+    for unflat_param_state in unflat_param_states:
+        if unflat_param_state is None:
+            continue
+        if state_names is None:
+            state_names = set(unflat_param_state.keys())
+        else:
+            if state_names != set(unflat_param_state.keys()):
+                raise ValueError(
+                    "Differing optimizer state names for the unflattened "
+                    f"parameters: {unflat_param_names}"
+                )
+    assert state_names is not None
+
+    # Flatten the state
+    flat_state: Dict[str, Any] = {}
+    for state_name in state_names:
+        state_values = [
+            unflat_param_state[state_name] if unflat_param_state is not None else None
+            for unflat_param_state in unflat_param_states
+        ]
+        non_none_state_values = [v for v in state_values if v is not None]
+        # If all ranks have None, this is a None value
+        if not non_none_state_values:
+            flat_state[state_name] = None
+            continue
+        are_pos_dim_tensors = are_zero_dim_tensors = are_non_tensors = True
+        for v in non_none_state_values:
+            are_pos_dim_tensors &= torch.is_tensor(v) and v.dim() > 0
+            are_zero_dim_tensors &= _is_zero_dim_tensor(v)
+            are_non_tensors &= not torch.is_tensor(v)
+        types = {type(v) for v in non_none_state_values}
+        if len(types) != 1 or not (
+            are_pos_dim_tensors or are_zero_dim_tensors or are_non_tensors
+        ):
+            raise ValueError(
+                f"Differing optimizer state types for state {state_name}, "
+                f"values {non_none_state_values}, and unflattened parameter "
+                f"names {unflat_param_names}"
+            )
+        if are_pos_dim_tensors:
+            flat_tensor = _flatten_tensor_optim_state(
+                state_name,
+                state_values,
+                unflat_param_names,
+                unflat_param_shapes,
+                handle,
+            )
+            # Shard the flattened tensor immediately to minimize max memory
+            # usage
+            if (
+                fsdp_state.world_size != 1
+                and fsdp_state.sharding_strategy != ShardingStrategy.NO_SHARD
+            ):
+                sharded_flat_tensor, _ = FlatParamHandle._get_shard(
+                    flat_tensor,
+                    fsdp_state.rank,
+                    fsdp_state.world_size,
+                )
+            else:
+                sharded_flat_tensor = flat_tensor
+            flat_state[state_name] = sharded_flat_tensor
+        elif are_zero_dim_tensors:
+            flat_state[state_name] = _flatten_zero_dim_tensor_optim_state(
+                state_name,
+                state_values,
+                unflat_param_names,
+            )
+        else:
+            assert are_non_tensors
+            flat_state[state_name] = _flatten_non_tensor_optim_state(
+                state_name,
+                state_values,
+                unflat_param_names,
+            )
+
+    return flat_state
+
+
+def _flatten_tensor_optim_state(
+    state_name: str,
+    pos_dim_tensors: List[torch.Tensor],
+    unflat_param_names: List[str],
+    unflat_param_shapes: Sequence[torch.Size],
+    handle: FlatParamHandle,
+) -> torch.Tensor:
+    """
+    Flattens the positive-dimension tensor optimizer state given by the values
+    ``tensors`` for the state ``state_name`` for a single flat parameter
+    from ``handle`` corresponding to the unflattened parameter names
+    ``unflat_param_names`` and unflatted parameter shapes
+    ``unflat_param_shapes``. This flattens each unflattened parameter's tensor
+    state into one tensor.
+
+    NOTE: We use zero tensors for any unflattened parameters without state
+    since some value is required to fill those entries. This assumes that the
+    zero tensor is mathematically equivalent to having no state, which is true
+    for Adam's "exp_avg" and "exp_avg_sq" but may not be true for all
+    optimizers.
+
+    Args:
+        state_name (str): Optimizer state name.
+        pos_dim_tensors (List[torch.Tensor]): Positive-dimension tensor
+            optimizer state values for the unflattened parameters corresponding
+            to the single flat parameter.
+        unflat_param_names (List[str]): A :class:`list` of unflattened
+            parameter names corresponding to the single flat parameter.
+        unflat_param_shapes (List[torch.Size]): Unflattened parameter shapes
+            corresponding to the single flat parameter.
+        handle (FlatParamHandle): The flat parameter's handle.
+
+    Returns:
+        torch.Tensor: A flat tensor containing the optimizer state
+        corresponding to ``state_name`` constructed by concatenating the
+        unflattened parameter tensor states in ``pos_dim_tensors`` (using zero
+        tensors for any unflattened parameters without the state).
+    """
+    flat_param = handle.flat_param
+    non_none_tensors = [t for t in pos_dim_tensors if t is not None]
+    # Check that all are tensors with the same dtype
+    dtypes = {t.dtype for t in non_none_tensors}
+    if len(dtypes) != 1:
+        raise ValueError(
+            "All unflattened parameters comprising a single flat "
+            "parameter must have positive-dimension tensor state with the "
+            f"same dtype but got dtypes {dtypes} for state {state_name} and "
+            f"unflattened parameter names {unflat_param_names}"
+        )
+    dtype = next(iter(dtypes))
+    # Check that each tensor state matches its parameter's shape
+    for tensor, shape in zip(pos_dim_tensors, unflat_param_shapes):
+        if tensor is None and len(shape) == 0:
+            raise ValueError("Flattening a zero-dimension parameter is not supported")
+        elif tensor is not None and tensor.shape != shape:
+            raise ValueError(
+                "Tensor optimizer state does not have same shape as its "
+                f"parameter: {tensor.shape} {shape}"
+            )
+    # Flatten the tensor states: we do not need to add any right-hand-side
+    # padding since the flat optimizer state tensor is sharded via
+    # `_get_shard()`, which pads the shard as needed (just like for the flat
+    # parameter)
+    cpu_device = torch.device("cpu")
+    tensors_to_flatten = [
+        torch.flatten(state_value.to(cpu_device))
+        if state_value is not None
+        else torch.flatten(
+            torch.zeros(
+                size=shape,
+                dtype=dtype,
+                device=cpu_device,
+            )
+        )
+        for state_value, shape in zip(pos_dim_tensors, unflat_param_shapes)
+    ]
+    flat_tensor = handle.flatten_tensors(tensors_to_flatten, handle._aligned_numel)
+    flat_param_shape = flat_param._unpadded_unsharded_size  # type: ignore[attr-defined]
+    assert flat_tensor.shape == flat_param_shape, (
+        f"tensor optim state: {flat_tensor.shape} "
+        f"flat parameter: {flat_param_shape}"
+    )
+    return flat_tensor
+
+
+def _flatten_zero_dim_tensor_optim_state(
+    state_name: str,
+    zero_dim_tensors: List[torch.Tensor],
+    unflat_param_names: List[str],
+) -> torch.Tensor:
+    """
+    Flattens the zero-dimension tensor optimizer state given by the values
+    ``zero_dim_tensors`` for the state ``state_name`` for a single flat
+    parameter corresponding to the unflattened parameter names
+    ``unflat_param_names`` by enforcing that all tensors are the same and using
+    that common value.
+
+    NOTE: The requirement that the tensors are the same across all unflattened
+    parameters comprising the flat parameter is needed to maintain the
+    invariant that FSDP performs the same computation as its non-sharded
+    equivalent. This means that none of the unflattened parameters can be
+    missing this state since imposing a value may differ from having no value.
+    For example, for Adam's "step", no value means maximum bias correction,
+    while having some positive value means less bias correction.
+
+    Args:
+        state_name (str): Optimizer state name.
+        zero_dim_tensors (List[torch.Tensor]): Zero-dimension optimizer state
+            for the unflattened parameters corresponding to the single
+            flat parameter.
+        unflat_param_names (List[str]): A :class:`list` of unflattened
+            parameter names corresponding to the single flat parameter.
+
+    Returns:
+        torch.Tensor: A zero-dimensional tensor giving the value of the state
+        ``state_name`` for all unflattened parameters corresponding to the
+        names ``unflat_param_names``.
+    """
+    non_none_tensors = [t for t in zero_dim_tensors if t is not None]
+    # Enforce that all have the same value and dtype
+    values_set = {t.item() if t is not None else None for t in zero_dim_tensors}
+    dtypes = {t.dtype if t is not None else None for t in zero_dim_tensors}
+    if (
+        len(non_none_tensors) != len(zero_dim_tensors)
+        or len(values_set) != 1
+        or len(dtypes) != 1
+    ):
+        raise ValueError(
+            "All unflattened parameters comprising a single flat "
+            "parameter must have scalar state with the same value and dtype "
+            f"but got values {values_set} and dtypes {dtypes} for state "
+            f"{state_name} and unflattened parameter names "
+            f"{unflat_param_names}"
+        )
+    value = next(iter(values_set))
+    dtype = next(iter(dtypes))
+    return torch.tensor(value, dtype=dtype, device=torch.device("cpu"))
+
+
+def _flatten_non_tensor_optim_state(
+    state_name: str,
+    non_tensors: List[Any],
+    unflat_param_names: List[str],
+) -> Any:
+    """
+    Flattens the non-tensor optimizer state given by the values ``non_tensors``
+    for the state ``state_name`` for a single flat parameter corresponding
+    to the unflattened parameter names ``unflat_param_names`` by enforcing that
+    all values are the same and using that common value.
+
+    See the note in :func:`_flatten_zero_dim_tensor_optim_state`.
+
+    Args:
+        state_name (str): Optimizer state name.
+        non_tensors (List[Any]): Non-tensor optimizer state for the unflattened
+            parameters corresponding to the single flat parameter.
+        unflat_param_names (List[str]): A :class:`list` of unflattened
+            parameter names corresponding to the single flat parameter.
+
+    Returns:
+        Any: A non-tensor giving the value of the state ``state_name`` for all
+        unflattened parameters corresponding to the names
+        ``unflat_param_names``.
+    """
+    non_none_non_tensors = [nt for nt in non_tensors if nt is not None]
+    # Enforce that all have the same value (same type already checked)
+    non_tensor_set = set(non_tensors)
+    if len(non_none_non_tensors) != len(non_tensors) or len(non_tensor_set) != 1:
+        raise ValueError(
+            "All unflattened parameters comprising a single flat "
+            "parameter must have scalar state with the same value and dtype "
+            f"but got values {non_tensor_set} for state {state_name} and  "
+            f"unflattened parameter names {unflat_param_names}"
+        )
+    non_tensor = next(iter(non_tensor_set))
+    return non_tensor
+
+
+def _rekey_sharded_optim_state_dict(
+    sharded_osd: Dict[str, Any],
+    model: nn.Module,
+    optim: torch.optim.Optimizer,
+    optim_input: Optional[
+        Union[
+            List[Dict[str, Any]],
+            Iterable[nn.Parameter],
+        ]
+    ],
+    using_optim_input: bool,
+    is_named_optimizer: bool = False,
+) -> Dict[str, Any]:
+    """
+    Rekeys the optimizer state dict from unflattened parameter names to flat
+    parameter IDs according to the calling rank's ``optim``, which may be
+    different across ranks. In particular, the unflattened parameter names are
+    represented as :class:`_OptimStateKey` s.
+    """
+    param_to_fqns = _get_param_to_fqns(model)
+    flat_param_to_fqn = _get_flat_param_to_fqn(model)
+    param_to_param_key: Dict[nn.Parameter, Union[int, str]] = cast(
+        Dict[nn.Parameter, Union[int, str]],
+        (
+            _get_param_to_param_id_from_optim_input(model, optim_input)
+            if using_optim_input
+            else _get_param_to_param_key(
+                optim, model, is_named_optimizer, param_to_fqns, flat_param_to_fqn
+            )
+        ),
+    )
+    # All parameter keys in `param_to_param_key` should be in
+    # `param_to_fqns` -- strict inequality follows when not all parameters are
+    # passed to the optimizer
+    assert len(param_to_param_key) <= len(param_to_fqns)
+
+    unflat_param_names_to_flat_param_key: Dict[
+        Tuple[str, ...], Union[int, str]
+    ] = {}  # for "state"
+    unflat_param_name_to_flat_param_key: Dict[
+        str, Union[int, str]
+    ] = {}  # for "param_groups"
+    for param, unflat_param_names in param_to_fqns.items():
+        if param not in param_to_param_key:
+            # This parameter was not passed to the optimizer
+            continue
+        flat_param_key = param_to_param_key[param]
+        unflat_param_names_to_flat_param_key[tuple(unflat_param_names)] = flat_param_key
+        for unflat_param_name in unflat_param_names:
+            unflat_param_name_to_flat_param_key[unflat_param_name] = flat_param_key
+
+    sharded_osd_state = sharded_osd["state"]
+    rekeyed_osd_state: Dict[Union[str, int], Any] = {}
+    for key, param_state in sharded_osd_state.items():
+        if isinstance(key, str):
+            rekeyed_osd_state[key] = param_state
+            continue
+        flat_param_key = unflat_param_names_to_flat_param_key.get(
+            key.unflat_param_names, key.unflat_param_names
+        )
+        rekeyed_osd_state[flat_param_key] = param_state
+
+    # Only process param_groups if it exists in sharded_osd
+    if "param_groups" in sharded_osd:
+        rekeyed_osd_param_groups: List[Dict[str, Any]] = []
+        for unflat_param_group in sharded_osd["param_groups"]:
+            flat_param_group = copy.deepcopy(unflat_param_group)
+            flat_param_keys = sorted(
+                {
+                    unflat_param_name_to_flat_param_key[unflat_param_name]
+                    for unflat_param_name in unflat_param_group["params"]
+                }
+            )
+            flat_param_group["params"] = flat_param_keys
+            rekeyed_osd_param_groups.append(flat_param_group)
+        return {"state": rekeyed_osd_state, "param_groups": rekeyed_osd_param_groups}
+    else:
+        return {"state": rekeyed_osd_state}
+
+
+def _get_param_id_to_param_from_optim_input(
+    model: nn.Module,
+    optim_input: Optional[
+        Union[
+            List[Dict[str, Any]],
+            Iterable[nn.Parameter],
+        ]
+    ] = None,
+) -> Dict[int, nn.Parameter]:
+    """
+    Constructs a mapping from parameter IDs to parameters. This may be used
+    both for models with ``FlatParameter`` s and without.
+
+    NOTE: This method is only preserved for backward compatibility. The method
+    :meth:`_get_param_key_to_param` is the preferred code path that does not
+    rely on ``optim_input``.
+
+    NOTE: We critically assume that, whether the optimizer input is a list of
+    parameters or a list of parameter groups, :class:`torch.optim.Optimizer`
+    enumerates the parameter IDs in order. In other words, for a parameter list
+    input, the parameter IDs should be in that list order, and for a parameter
+    groups input, the parameter IDs should be in order within each parameter
+    group and in order across parameter groups.
+
+    Args:
+        model (nn.Module): Model whose parameters are passed into the
+            optimizer.
+        optim_input (Optional[Union[List[Dict[str, Any]],
+        Iterable[nn.Parameter]]]): Input passed into the optimizer
+            representing either a :class:`list` of parameter groups or an
+            iterable of parameters; if ``None``, then this method assumes the
+            input was ``model.parameters()``. (Default: ``None``)
+
+    Returns:
+        List[nn.Parameter]: Mapping from parameter IDs to parameters,
+        where the parameter ID is implicitly the index in the :class:`list`.
+    """
+    # Assume the standard case of passing `model.parameters()` to the optimizer
+    # if `optim_input` is not specified
+    if optim_input is None:
+        return dict(enumerate(model.parameters()))
+    try:
+        params = cast(List[nn.Parameter], list(optim_input))
+    except TypeError as e:
+        raise TypeError(
+            "Optimizer input should be an iterable of Tensors or dicts, "
+            f"but got {optim_input}"
+        ) from e
+    if len(params) == 0:
+        raise ValueError("Optimizer input should not be empty")
+
+    # Check if the optimizer input represents tensors or parameter groups
+    all_tensors = True
+    all_dicts = True
+    for param in params:
+        all_tensors &= isinstance(param, torch.Tensor)
+        all_dicts &= isinstance(param, dict)
+    if not all_tensors and not all_dicts:
+        raise TypeError("Optimizer input should be an iterable of Tensors or dicts")
+    if all_tensors:
+        return dict(enumerate(params))
+    assert all_dicts
+    param_id_to_param: List[nn.Parameter] = []
+    for param_group in params:
+        has_params_key = "params" in param_group  # type: ignore[operator]
+        assert has_params_key, (
+            'A parameter group should map "params" to a list of the '
+            "parameters in the group"
+        )
+        # Implicitly map `flat_param_id` (current length of the list) to
+        # `param`
+        param_id_to_param.extend(param_group["params"])  # type: ignore[index]
+    return dict(enumerate(param_id_to_param))
+
+
+def _get_flat_param_to_fqn(model: torch.nn.Module) -> Dict[FlatParameter, str]:
+    """
+    Constructs a mapping from ``FlatParameter`` to a cleaned (devoid of prefixes
+    from wrappers) fully qualified name (FQN). Note that this FQN is "non-canonical"
+    because ``FlatParameter``  s do not come from the original module but are
+    registered only after FSDP has been applied. This function returns the FSDP-given
+    name for the ``FlatParameter`` (usually module._flat_param) as opposed to the
+    canonical FQNs returned for ``FlatParameter`` s in ``_common_utils._get_param_to_fqns(...)``).
+
+    Consequently, this function will only return a non-empty mapping if FSDP was
+    applied with ``use_orig_params=False`` as, otherwise, the original parameters
+    are used within the module and there would be no ``FlatParameter`` s in the module.
+
+    """
+
+    def module_fn(module, prefix, tree_level, flat_param_to_fqn):
+        for param_name, param in _named_parameters_with_duplicates(
+            module, recurse=False
+        ):
+            if not isinstance(param, FlatParameter):
+                continue
+            fqn = clean_tensor_name(prefix + param_name)
+            flat_param_to_fqn[param] = fqn
+
+    def return_fn(flat_param_to_fqn):
+        return flat_param_to_fqn
+
+    flat_param_to_fqn_ret: Dict[FlatParameter, str] = {}
+    return _apply_to_modules(
+        model,
+        module_fn,
+        return_fn,
+        [fqn for fqn, _ in _named_parameters_with_duplicates(model)],
+        flat_param_to_fqn_ret,
+    )
+
+
+def _get_param_key_to_param(
+    optim: torch.optim.Optimizer,
+    model: Optional[nn.Module] = None,
+    is_named_optimizer: bool = False,
+    param_to_fqns: Optional[Dict[nn.Parameter, List[str]]] = None,
+    flat_param_to_fqn: Optional[Dict[FlatParameter, str]] = None,
+) -> Dict[Union[int, str], nn.Parameter]:
+    """
+    Constructs a mapping from parameter keys to parameters. For the regular
+    optimizers, the keys are parameter IDs. For NamedOptimizer, the keys
+    are FQNs. This API may be used both for models with ``FlatParameter`` s and
+    without.
+    """
+    clean_fqn_to_curr_fqn: Dict[str, str] = {}
+    if is_named_optimizer:
+        assert (
+            param_to_fqns is not None and flat_param_to_fqn is not None
+        ), "The optimizer is a NamedOptimizer, `param_to_fqns` must not be None."
+        assert model is not None
+        for key, _ in _named_parameters_with_duplicates(model):
+            clean_fqn_to_curr_fqn[clean_tensor_name(key)] = key
+
+    param_key_to_param: Dict[Union[str, int], nn.Parameter] = {}
+    pid = 0
+    for param_group in optim.param_groups:
+        if is_named_optimizer:
+            for param in param_group["params"]:
+                assert flat_param_to_fqn is not None
+                if param in flat_param_to_fqn:
+                    # FlatParameter case
+                    key = flat_param_to_fqn[param]
+                else:
+                    assert param_to_fqns is not None
+                    # use_orig_params case
+                    assert len(param_to_fqns[param]) == 1
+                    key = param_to_fqns[param][0]
+                try:
+                    key = clean_fqn_to_curr_fqn[key]
+                except KeyError as e:
+                    raise KeyError(
+                        f"Can't find {key} from {list(clean_fqn_to_curr_fqn.keys())}."
+                    ) from e
+                param_key_to_param[key] = param
+        else:
+            for param in param_group["params"]:
+                param_key_to_param[pid] = param
+                pid += 1
+
+    return param_key_to_param
+
+
+def _get_param_to_param_key(
+    optim: torch.optim.Optimizer,
+    model: Optional[nn.Module] = None,
+    is_named_optimizer: bool = False,
+    param_to_fqns: Optional[Dict[nn.Parameter, List[str]]] = None,
+    flat_param_to_fqn: Optional[Dict[FlatParameter, str]] = None,
+) -> Dict[nn.Parameter, Union[int, str]]:
+    """
+    Constructs the inverse mapping of :func:`_get_param_key_to_param`. This API
+    only supports the case where `optim` is a regular optimizer, not NamedOptimizer.
+    So the parameter keys will be parameter ids.
+    """
+    param_id_to_param = _get_param_key_to_param(
+        optim, model, is_named_optimizer, param_to_fqns, flat_param_to_fqn
+    )
+    return {param: param_id for param_id, param in param_id_to_param.items()}
+
+
+def _get_param_to_param_id_from_optim_input(
+    model: nn.Module,
+    optim_input: Optional[
+        Union[
+            List[Dict[str, Any]],
+            Iterable[nn.Parameter],
+        ]
+    ] = None,
+) -> Dict[nn.Parameter, int]:
+    """Constructs the inverse mapping of :func:`_get_param_id_to_param_from_optim_input`."""
+    param_id_to_param = _get_param_id_to_param_from_optim_input(model, optim_input)
+    return {param: param_id for param_id, param in param_id_to_param.items()}
+
+
+def _check_missing_keys_on_rank(
+    r0_optim_state_keys: List[_OptimStateKey],
+    optim_state_key_to_param_key: Dict[_OptimStateKey, Union[str, int]],
+    param_key_to_param: Dict[Union[str, int], nn.Parameter],
+    group: Optional[dist.ProcessGroup],
+) -> None:
+    # Ensure that all ranks have at least the optimizer states needed by
+    # rank 0's optimizer
+    missing_keys: List[_OptimStateKey] = []
+    for r0_optim_state_key in r0_optim_state_keys:
+        if r0_optim_state_key not in optim_state_key_to_param_key:
+            # A parameter from rank 0's optimizer does not exist for this
+            # rank's optimizer
+            missing_keys.append(r0_optim_state_key)
+            continue
+        param_key = optim_state_key_to_param_key[r0_optim_state_key]
+        if isinstance(param_key, int):
+            assert param_key >= 0 and param_key < len(
+                param_key_to_param
+            ), "Check the `param_key_to_param` construction"
+    # We cannot use FSDPState.compute_device as this API is a global view.
+    device = _get_pg_default_device(group)
+    num_missing = torch.tensor([len(missing_keys)], dtype=torch.int32, device=device)
+    dist.all_reduce(num_missing, group=group)
+    if num_missing.item() > 0:
+        obj_list = [None for _ in range(dist.get_world_size(group))]
+        dist.all_gather_object(obj_list, missing_keys, group=group)
+        error_msg = (
+            "FSDP currently requires each rank to have at least the "
+            "optimizer states needed by rank 0's optimizer but some ranks "
+            "are missing some of those states"
+        )
+        for rank, keys in enumerate(obj_list):
+            keys = cast(List[_OptimStateKey], keys)
+            if len(keys) > 0:
+                error_msg += (
+                    f"\nRank {rank} is missing states for the parameters: "
+                    f"{[key.unflat_param_names for key in keys]}"
+                )
+        raise RuntimeError(error_msg)
+
+
+def _map_param_key_to_optim_keys(
+    optim_state_dict: Dict[str, Any],
+    group: Optional[dist.ProcessGroup],
+    param_key_to_param: Dict[Union[int, str], nn.Parameter],
+    param_to_fqns: Dict[nn.Parameter, List[str]],
+    fqn_to_fsdp_param_info: Dict[str, FSDPParamInfo],
+    merge_keys: bool = False,
+) -> Tuple[List[_OptimStateKey], Dict[_OptimStateKey, Union[int, str]]]:
+    """
+    Construct the local mapping between the ``_OptimStateKey`` and parameter keys
+    and all the ``_OptimStateKey`` across ranks. If ``merge_keys`` is False, rank0
+    must contain all the ``_OptimStateKey``, an exception will be raised otherwise.
+    Note that ``merge_keys`` should equal to ``use_orig_params``.
+    """
+    rank = dist.get_rank(group)
+    optim_state_key_to_param_key: Dict[_OptimStateKey, Union[int, str]] = {}  # local
+    all_optim_state_keys: List[_OptimStateKey] = []
+
+    for param_key, param in param_key_to_param.items():
+        # Do not include parameters without state to avoid empty mappings
+        # just like in normal `torch.optim.Optimizer.state_dict()`
+        if param_key not in optim_state_dict["state"]:
+            continue
+        fqns = param_to_fqns[param]
+        is_fsdp_managed = isinstance(param, FlatParameter)
+        if is_fsdp_managed:
+            assert fqns[0] in fqn_to_fsdp_param_info, (
+                fqns[0],
+                list(fqn_to_fsdp_param_info.keys()),
+            )
+        is_fsdp_managed = fqns[0] in fqn_to_fsdp_param_info
+        optim_state_key = _OptimStateKey(
+            unflat_param_names=tuple(fqns),
+            is_fsdp_managed=is_fsdp_managed,
+        )
+        if rank == 0 or merge_keys:
+            all_optim_state_keys.append(optim_state_key)
+        optim_state_key_to_param_key[optim_state_key] = param_key
+
+    if merge_keys:
+        all_keys: List[List[_OptimStateKey]] = [
+            [] for _ in range(dist.get_world_size(group))
+        ]
+        dist.all_gather_object(all_keys, all_optim_state_keys, group=group)
+        merge_all_optim_state_keys = [
+            key for local_keys in all_keys for key in local_keys
+        ]
+        all_optim_state_keys = sorted(set(merge_all_optim_state_keys))
+    else:
+        key_obj_list: List[Optional[List[_OptimStateKey]]] = (
+            [all_optim_state_keys] if rank == 0 else [None]
+        )
+        dist.broadcast_object_list(key_obj_list, src=0, group=group)
+        assert key_obj_list[0] is not None
+        all_optim_state_keys = key_obj_list[0]
+        _check_missing_keys_on_rank(
+            all_optim_state_keys,
+            optim_state_key_to_param_key,
+            param_key_to_param,
+            group,
+        )
+
+    return all_optim_state_keys, optim_state_key_to_param_key
+
+
+def _unflatten_param_groups(
+    state_dict: Dict[str, Any],
+    param_key_to_param: Dict[Union[int, str], nn.Parameter],
+    param_to_fqns: Dict[nn.Parameter, List[str]],
+) -> List[Dict[str, Any]]:
+    param_groups: List[Dict[str, Any]] = []
+    for flat_param_group in state_dict["param_groups"]:
+        unflat_param_group = copy.deepcopy(flat_param_group)
+        param_group_params = [
+            param_key_to_param[flat_param_key]
+            for flat_param_key in flat_param_group["params"]
+        ]
+        nested_unflat_param_names = [
+            param_to_fqns[param] for param in param_group_params
+        ]
+        unflat_param_group["params"] = [
+            unflat_param_name
+            for unflat_param_names in nested_unflat_param_names
+            for unflat_param_name in unflat_param_names
+        ]  # flatten the list of lists
+        param_groups.append(unflat_param_group)
+    return param_groups
+
+
+def _is_named_optimizer(optim_state_dict: Dict[str, Any]) -> bool:
+    """
+    Returns whether the state_dict is from a NamedOptimizer.
+    This function checks that the keys in the state_dict['state'] are strings
+    (which usually are FQNs) versus integers (which usually refer to param_ids
+    from a vanilla torch.optim.Optimizer).
+    """
+    state = optim_state_dict.get("state", None)
+    if not state:
+        # If we cannot find a state, assume it is not NamedOptimizer as
+        # NamedOptimizer has eager initialization.
+        return False
+    try:
+        key = next(iter(state.keys()))
+    except Exception as e:
+        raise Exception(optim_state_dict) from e  # noqa: TRY002
+    return isinstance(key, str)
+
+
+@dataclass
+class StateInfo:
+    # The key of these dictionaries are the state name, e.g., `exp_avg`.
+    tensors: Dict[str, _PosDimTensorInfo]
+    scalar_tensors: Dict[str, torch.Tensor]
+    non_tensors: Dict[str, Any]
+
+
+def _allgather_state_info(
+    fsdp_state: _FSDPState,
+    input_states: Dict[str, Any],
+) -> List[Dict[str, StateInfo]]:
+    """
+    Given the ``input_states``, allgather StateInfo for each state. The function
+    uses all_gather_object to gather StateInfo so no GPU tensors are sent.
+    """
+
+    processed_state_dict: Dict[str, StateInfo] = {}
+    gathered_state_info: List[Dict[str, StateInfo]] = [
+        {} for _ in range(fsdp_state.world_size)
+    ]
+
+    for fqn, optim_state in input_states.items():
+        # Allgather the scalar tensor state, non-tensor states and tensors metadata.
+        processed_state = StateInfo({}, {}, {})
+        for state_name, value in sorted_items(optim_state):
+            if torch.is_tensor(value):
+                if value.dim() == 0:
+                    # Ensure that `step` is on CPU.
+                    processed_state.scalar_tensors[state_name] = value.cpu()
+                else:
+                    processed_state.tensors[state_name] = _PosDimTensorInfo(
+                        value.shape, value.dtype
+                    )
+            else:
+                processed_state.non_tensors[state_name] = value
+        processed_state_dict[fqn] = processed_state
+    dist.all_gather_object(
+        gathered_state_info,
+        processed_state_dict,
+        group=fsdp_state.process_group,
+    )
+    return gathered_state_info
+
+
+def _convert_all_state_info(
+    fsdp_param_info: FSDPParamInfo,
+    gathered_state_info: List[Dict[str, StateInfo]],
+    input_states: Dict[str, Any],
+    output_states: Dict[str, Dict[str, Any]],
+) -> Tuple[Optional[torch.dtype], Dict[str, List[Optional[torch.Tensor]]]]:
+    """
+    Given the ``gathered_state_info`` and ``input_states``, the API converted
+    the StateInfo into the original state if the state is not a non-scalar
+    tensor. For a multi-dimensional tensor, the local state will be stored in
+    ``state_buffer`` in a correct order for later allgather purpose.
+    """
+
+    state_buffers: Dict[str, List[Optional[torch.Tensor]]] = {}
+
+    for fqn, gathered_state in output_states.items():
+        state_info = [s[fqn] for s in gathered_state_info]
+        all_tensor_states = sorted(
+            {n for state in state_info for n in state.tensors.keys()}
+        )
+        empty_ranks: Set[int] = set()
+        dtype: Optional[torch.dtype] = None
+        # First check all the non-scalar states and get the information of
+        # states on each rank.
+        for state_name in all_tensor_states:
+            numels = []
+            _empty_ranks: Set[int] = set()
+            for rank, object_state in enumerate(state_info):
+                numels.append(0)
+                info = object_state.tensors.get(state_name, None)
+                if info is not None:
+                    numels[-1] = info.shape.numel()
+                    if not dtype:
+                        dtype = info.dtype
+                    else:
+                        assert dtype == info.dtype
+                if numels[-1] == 0:
+                    _empty_ranks.add(rank)
+
+            assert not empty_ranks or empty_ranks == _empty_ranks
+            empty_ranks = _empty_ranks
+            if state_name not in state_buffers:
+                state_buffers[state_name] = [
+                    None for _ in fsdp_param_info.param_indices
+                ]
+            local_state = input_states[fqn].get(state_name, None)
+            # N.B. We need to move the state to compute_device. The reason is
+            # not yet clear and we need to figure out why the state may be on a
+            # different device.
+            if local_state is not None:
+                local_state = local_state.to(fsdp_param_info.state.compute_device)
+            state_buffers[state_name][fsdp_param_info.param_indices[fqn]] = local_state
+
+        # Restoring the scalar and non-tensor states. If the corresponding
+        # non-scalar states do not exist on the rank, we also skip the scalar
+        # non-tensor states on that rank.
+        for rank, object_state in enumerate(state_info):
+            if rank in empty_ranks:
+                continue
+            for name, non_tensor_value in object_state.non_tensors.items():
+                curr_non_tensor_value = gathered_state.get(name, None)
+                assert (
+                    curr_non_tensor_value is None
+                    or curr_non_tensor_value == non_tensor_value
+                ), (
+                    f"Rank {rank} has different values for {name}: {non_tensor_value}."
+                    + f" Other ranks: {curr_non_tensor_value}"
+                )
+                gathered_state[name] = non_tensor_value
+
+            for name, scalar_tensor_value in object_state.scalar_tensors.items():
+                curr_scalar_tensor_value = gathered_state.get(name, None)
+                assert curr_scalar_tensor_value is None or torch.equal(
+                    scalar_tensor_value, curr_scalar_tensor_value
+                ), (
+                    f"Rank {rank} has different values for {name}: {scalar_tensor_value}."
+                    + f" Other ranks: {curr_scalar_tensor_value}"
+                )
+                gathered_state[name] = scalar_tensor_value
+
+    return dtype, state_buffers  # type: ignore[possibly-undefined]
+
+
+def _unflatten_orig_param_states(
+    fsdp_param_info: FSDPParamInfo,
+    output_states: Dict[str, Dict[str, Any]],
+    state_name: str,
+    shard_state: bool,
+    to_save: bool,
+    cpu_offload: bool,
+) -> None:
+    """
+    Given a output state dict, ``output_states``, which the keys are FQNs to the
+    original parameters (not FlatParameters nor parmeter ID), and the values
+    are gathered states, unflatten the states to the original dimensions.
+
+    This function performs the unflattening process in-place.
+    """
+    if not to_save:
+        return
+    flat_param = fsdp_param_info.handle.flat_param
+    fsdp_state = fsdp_param_info.state
+    for fqn, gathered_state in output_states.items():
+        value = gathered_state[state_name]
+        param_idx = fsdp_param_info.param_indices[fqn]
+
+        # TODO: This solution is not general and only apply to PTD TP solution.
+        if isinstance(value, DTensor):
+            placement = value.placements[0]
+            # If gathered state is a DTensor and its TP placement is not Replicate(), we need to
+            # gather the tensor on its TP dimension before chunking them into DTensor again.
+            if placement != Replicate():
+                placement_dim = placement.dim  # type: ignore[attr-defined]
+                value_local = value.redistribute(placements=(Replicate(),))
+                reshape_size = list(flat_param._shapes[param_idx])
+                reshape_size[placement_dim] *= value.device_mesh.size(0)
+                reshape_size = torch.Size(reshape_size)
+                value = value.reshape(reshape_size)
+            # If gathered state is a replicate DTensor, we directly reshape it.
+            else:
+                value = value.reshape(flat_param._shapes[param_idx])
+        else:
+            # If gathered state is a tensor, we directly reshape it into unflatten state.
+            value = value.reshape(flat_param._shapes[param_idx])
+
+        if shard_state:
+            osd_config = fsdp_state._optim_state_dict_config
+            if getattr(osd_config, "_use_dtensor", False):
+                assert fsdp_state._device_mesh is not None
+                value = _ext_chunk_dtensor(
+                    value,
+                    fsdp_state.rank,
+                    fsdp_state._device_mesh,
+                    fsdp_state._fsdp_extension,
+                )
+            else:
+                assert fsdp_state.process_group is not None
+                value = _ext_chunk_tensor(
+                    value,
+                    fsdp_state.rank,
+                    fsdp_state.world_size,
+                    fsdp_state._device_handle.device_count(),
+                    fsdp_state.process_group,
+                    fsdp_state._fsdp_extension,
+                )
+        elif not cpu_offload:
+            with SimpleProfiler.profile("clone"):
+                value = value.detach().clone()
+
+        if cpu_offload:
+            with SimpleProfiler.profile(SimpleProfiler.Type.D2H):
+                value = value.cpu()
+        gathered_state[state_name] = value
+
+
+def _allgather_orig_param_states(
+    fsdp_param_info: FSDPParamInfo,
+    gathered_state_info: List[Dict[str, StateInfo]],
+    input_states: Dict[str, Any],
+    shard_state: bool,
+    to_save: bool,
+    cpu_offload: bool,
+) -> Dict[str, Dict[str, Any]]:
+    """
+    Given the ``gathered_state_info`` and ``input_states``, the API allgathers
+    all tensor states and restore non-tensor states from ``gathered_state_info``.
+    """
+    fsdp_state = fsdp_param_info.state
+    if fsdp_state.rank == 0 and dist.get_debug_level() == dist.DebugLevel.DETAIL:
+        logger.info(
+            "Memory Summary before calling to _allgather_orig_param_states %s",
+            fsdp_state._device_handle.memory_summary(),
+        )
+
+    output_states: Dict[str, Dict[str, Any]] = {fqn: {} for fqn in input_states.keys()}
+
+    dtype, state_buffers = _convert_all_state_info(
+        fsdp_param_info, gathered_state_info, input_states, output_states
+    )
+
+    if len(state_buffers) == 0:
+        return output_states
+
+    has_state_params: List[bool] = [
+        True if fqn in output_states else False
+        for fqn, idx in fsdp_param_info.param_indices.items()
+    ]
+
+    # Loop through the ``state_buffers`` and construct the flattened, concatenated,
+    # sharded states. The size of the constructed state will be the same size as
+    # flat_param (also sharded).
+    # Then we perform an allgather_into_tensor to get the full flat_param state.
+    # The full flat_param state is the result of concatenation of multiple states
+    # the order of of flat_param._fqns.
+    # The final step is to split the flat_param state into original param states
+    # and return the result.
+    flat_param = fsdp_param_info.handle.flat_param
+    empty_func = functools.partial(
+        torch.empty, dtype=dtype, device=fsdp_state.compute_device
+    )
+    gathered_tensor = empty_func(flat_param._padded_unsharded_size)
+    # Synchronize can be slow but this will be easier for us to debug.
+    fsdp_state._device_handle.synchronize()
+    for state_name, buffers in state_buffers.items():
+        local_buffers: List[torch.Tensor] = []
+        begin = fsdp_state.rank * flat_param._sharded_size.numel()
+        # End is inclusive.
+        end = begin + flat_param._sharded_size.numel() - 1
+        # param_idx corresponds to the parameter index in the FlatParameter.
+        mem_offset, param_idx = 0, 0
+        for numel, is_padding in zip(
+            flat_param._numels_with_padding, flat_param._is_padding_mask
+        ):
+            frozen_and_no_state = not is_padding and (
+                not fsdp_param_info.param_requires_grad[param_idx]
+                and not has_state_params[param_idx]
+            )
+
+            if is_padding or frozen_and_no_state:
+                # This memory range is a padding or the param is frozen and does
+                # not require gradient. For the later case, we treat it as a
+                # padding and add empty values to the local_buffers.
+
+                padding_begin, padding_end = mem_offset, mem_offset + numel - 1
+                if padding_begin <= begin <= padding_end:
+                    # The range is an align padding before the first parameter in
+                    # the shard. The shard includes parts of this align padding.
+                    padding_len = (
+                        padding_end - begin + 1
+                        if end >= padding_end
+                        else end - begin + 1
+                    )
+                elif padding_begin <= end <= padding_end:
+                    # The range is an align padding after the last parameter in
+                    # the shard. The shard includes parts of this align padding.
+                    padding_len = (
+                        end - padding_begin + 1
+                        if begin <= padding_begin
+                        else end - begin + 1
+                    )
+                elif begin < padding_begin <= padding_end < end:
+                    # The range is an align padding that is completely in the
+                    # shard.
+                    padding_len = numel
+                else:
+                    padding_len = 0
+                if padding_len:
+                    local_buffers.append(empty_func(padding_len))
+
+            if not is_padding:
+                # This memory range is a parameter in FlatParameter. So there
+                # should be an corresponding state in the optimizer unless the
+                # parameter is frozen, which we treat it as a padding above.
+
+                # We need to check if this rank owns the buffer. If this is None:
+                # 1.) the rank does not own any part of the original parameter.
+                #     As a result, there is no corresponding optimizer state on
+                #     the rank as well.
+                # 2.) the parameter is frozen AND no optimizer state for the
+                #     parameter. If a parameter is frozen, there can still be
+                #     optimizer state if the parameter is not frozen in the
+                #     previous steps.
+                if buffers[param_idx] is not None:
+                    local_buffers.append(cast(torch.Tensor, buffers[param_idx]))
+                param_idx += 1
+
+            mem_offset += numel
+
+        shard_numel_padded = flat_param._sharded_size.numel() - (
+            sum(t.numel() for t in local_buffers)
+        )
+
+        assert flat_param._shard_numel_padded == shard_numel_padded, (
+            "Manually calculated _sharded_numel_padded is incorrect. "
+            f"_shard_numel_padded={flat_param._shard_numel_padded}, "
+            f"shard_numel_padded={shard_numel_padded}, "
+            f"_sharded_size.numel={flat_param._sharded_size.numel()}, "
+            f"_numels_with_padding={flat_param._numels_with_padding}, "
+            f"begin={begin}, end={end},"
+        )
+        if shard_numel_padded > 0:
+            # Add right-handed padding.
+            local_buffers.append(empty_func(shard_numel_padded))
+        local_shard = torch.cat(local_buffers)
+        assert local_shard.numel() * fsdp_state.world_size == gathered_tensor.numel(), (
+            "The size of local shard times the world size should equal to the "
+            "gathered tensor size. The inconsistency may be from a bug of "
+            "FlatParameter's metadata or the reconstruction logic in optimizer "
+            "state dict."
+        )
+        fsdp_state._device_handle.synchronize()
+        with SimpleProfiler.profile(SimpleProfiler.Type.ALLGATHER):
+            dist.all_gather_into_tensor(
+                gathered_tensor, local_shard, group=fsdp_state.process_group
+            )
+            # Synchronize can be slow but this will be easier for us to debug.
+            fsdp_state._device_handle.synchronize()
+
+        unpadded_tensor = gathered_tensor[: flat_param._unpadded_unsharded_size.numel()]
+        flat_param_handle = fsdp_param_info.handle
+        orig_states = flat_param_handle._get_unflat_views_aligned(unpadded_tensor)
+        assert len(orig_states) == len(fsdp_param_info.param_indices), (
+            "The number of parameters from FlatParameter is not consistent to "
+            "the number of states used by optimizer state dict reconstruction "
+            "logic."
+        )
+        for fqn, idx in fsdp_param_info.param_indices.items():
+            if fsdp_param_info.param_requires_grad[idx] or fqn in output_states:
+                output_states[fqn][state_name] = orig_states[idx]
+
+        _unflatten_orig_param_states(
+            fsdp_param_info,
+            output_states,
+            state_name,
+            shard_state,
+            to_save,
+            cpu_offload,
+        )
+
+    del gathered_tensor
+    return output_states
+
+
+def _gather_all_orig_param_state(
+    fsdp_param_info: FSDPParamInfo,
+    input_states: Dict[str, Any],
+    shard_state: bool,
+    to_save: bool,
+    cpu_offload: bool,
+) -> Dict[str, Any]:
+    """
+    Given a optimizer state dict, ``input_states``, which the keys are FQNs to the
+    original parameters (not FlatParameters nor parmeter ID), gather all the
+    states and unflatten them to the original dimensions. Note that all the
+    params referred by the ``input_states`` must be managed by FSDP.
+    """
+    fsdp_state = fsdp_param_info.state
+    if (
+        fsdp_state.world_size == 1
+        or fsdp_state.sharding_strategy == ShardingStrategy.NO_SHARD
+    ):
+        return input_states if to_save else {}
+
+    with SimpleProfiler.profile(SimpleProfiler.Type.RESHARDING):
+        with SimpleProfiler.profile(SimpleProfiler.Type.ALLGATHER_OBJ):
+            gathered_state_info = _allgather_state_info(fsdp_state, input_states)
+        output_states = _allgather_orig_param_states(
+            fsdp_param_info,
+            gathered_state_info,
+            input_states,
+            shard_state,
+            to_save,
+            cpu_offload,
+        )
+    if to_save:
+        for key, idx in fsdp_param_info.param_indices.items():
+            if key in output_states:
+                continue
+            if not fsdp_param_info.param_requires_grad[idx]:
+                continue
+
+            raise RuntimeError(
+                f"{key} is not in the output state. "
+                "The FSDPParamInfo has the param keys "
+                f"{sorted(fsdp_param_info.param_indices.keys())} while "
+                "the output_states has the param keys "
+                f"{sorted(output_states.keys())}."
+            )
+        return output_states
+    else:
+        return {}
+
+
+def _convert_state_with_orig_params(
+    all_optim_state_keys: List[_OptimStateKey],
+    optim_state_key_to_param_key: Dict[_OptimStateKey, Union[int, str]],
+    fqn_to_fsdp_param_info: Dict[str, FSDPParamInfo],
+    optim_state_dict: Dict[Union[str, int], Any],
+    to_save: bool,
+    shard_state: bool,
+    cpu_offload: bool = True,
+) -> Dict[str, Any]:
+    fsdp_osd_state: Dict[str, Any] = {}
+    # This variable is used to deduplicate the FSDPParamInfo as one FSDPParamInfo
+    # usually corresponds to multiple parameters. We could not use FSDPParamInfo
+    # as the key because FSDPParamInfo is not hashable. As a result, we fall back
+    # to `id(FSDPParamInfo)`, which the type is an integer.
+    all_states: Dict[int, Dict[str, Any]] = {}
+    # Iterate in rank 0's flat parameter ID order to ensure aligned all-gathers
+    # across ranks
+    for optim_state_key in all_optim_state_keys:
+        param_key: Union[str, int, None] = optim_state_key_to_param_key.get(
+            optim_state_key, None
+        )
+
+        if param_key is None and not optim_state_key.is_fsdp_managed:
+            continue
+
+        if optim_state_key.is_fsdp_managed:
+            fqn = optim_state_key.unflat_param_names[0]
+            fsdp_param_info = fqn_to_fsdp_param_info.get(fqn, None)
+            if fsdp_param_info is None:
+                # This can happen if the not all FSDP instances have all the
+                # parameters. This can happen with FSDP + some MPMD style
+                # parallelism.
+
+                # TODO: it is unclear if we need to do the same check with
+                # non-FSDP managed keys.
+                continue
+            state = {} if param_key is None else optim_state_dict[param_key]
+            if id(fsdp_param_info) not in all_states:
+                all_states[id(fsdp_param_info)] = {}
+            all_states[id(fsdp_param_info)][fqn] = state
+
+        elif to_save:
+            assert len(optim_state_key.unflat_param_names) == 1
+            unflat_param_name = optim_state_key.unflat_param_names[0]
+            with SimpleProfiler.profile("none_fsdp_managed_copy"):
+                param_key = cast(Union[str, int], param_key)
+                fsdp_osd_state[unflat_param_name] = copy.copy(
+                    optim_state_dict[param_key]
+                )
+                if cpu_offload:
+                    for state_name, value in sorted_items(
+                        fsdp_osd_state[unflat_param_name]
+                    ):
+                        if not torch.is_tensor(value):
+                            continue
+                        fsdp_osd_state[unflat_param_name][state_name] = value.cpu()
+
+    # Instead of gathering the state of each parameter individually, we perform
+    # the gathering  all at once to speed up the process.
+    for _all_states in all_states.values():
+        fqn = next(iter(_all_states.keys()))
+        fsdp_param_info = fqn_to_fsdp_param_info[fqn]
+        assert len(fsdp_param_info.param_requires_grad) > 0, (
+            "With use_orig_params, FSDPParamInfo should have requires_grad "
+            "information. However, the length is zero."
+        )
+        for key, idx in fsdp_param_info.param_indices.items():
+            if key in _all_states:
+                continue
+            if not fsdp_param_info.param_requires_grad[idx]:
+                continue
+            raise RuntimeError(
+                f"{key} is not in the optimizer state. "
+                "The FSDPParamInfo has the param keys "
+                f"{sorted(fsdp_param_info.param_indices.keys())} while "
+                "the optimizer has the param keys "
+                f"{sorted(_all_states.keys())}."
+            )
+        fsdp_osd_state.update(
+            _gather_all_orig_param_state(
+                fsdp_param_info,
+                _all_states,
+                shard_state,
+                to_save,
+                cpu_offload,
+            )
+        )
+
+    return fsdp_osd_state
+
+
+def _convert_state_with_flat_params(
+    all_optim_state_keys: List[_OptimStateKey],
+    optim_state_key_to_param_key: Dict[_OptimStateKey, Union[int, str]],
+    fqn_to_fsdp_param_info: Dict[str, FSDPParamInfo],
+    optim_state_dict: Dict[Union[str, int], Any],
+    to_save: bool,
+    shard_state: bool,
+    cpu_offload: bool = True,
+) -> Dict[str, Any]:
+    fsdp_osd_state: Dict[str, Any] = {}
+    # Iterate in rank 0's flat parameter ID order to ensure aligned all-gathers
+    # across ranks
+    for optim_state_key in all_optim_state_keys:
+        param_key: Union[str, int, None] = optim_state_key_to_param_key.get(
+            optim_state_key, None
+        )
+
+        assert param_key is not None, (
+            "If use_orig_params is False, we must be able to find the "
+            f"corresponding param id. {optim_state_key} {param_key}"
+        )
+
+        if optim_state_key.is_fsdp_managed:
+            # If there are multiple unflat_param_names (not use_orig_params),
+            # they share the same FSDPParamInfo. So the first unflat_param_name
+            # is sufficient to fetch the FSDPParamInfo.
+            fqn = optim_state_key.unflat_param_names[0]
+            fsdp_param_info = fqn_to_fsdp_param_info[fqn]
+            unflat_state = _unflatten_optim_state(
+                fsdp_param_info,
+                optim_state_dict[param_key],
+                to_save,
+                shard_state,
+                cpu_offload,
+            )
+            if to_save:
+                assert len(unflat_state) == len(optim_state_key.unflat_param_names)
+                for unflat_param_name, unflat_param_state in zip(
+                    optim_state_key.unflat_param_names,
+                    unflat_state,
+                ):
+                    fsdp_osd_state[unflat_param_name] = unflat_param_state
+        elif to_save:
+            assert len(optim_state_key.unflat_param_names) == 1
+            unflat_param_name = optim_state_key.unflat_param_names[0]
+            fsdp_osd_state[unflat_param_name] = copy.copy(optim_state_dict[param_key])
+            if cpu_offload:
+                for state_name, value in sorted_items(
+                    fsdp_osd_state[unflat_param_name]
+                ):
+                    if not torch.is_tensor(value):
+                        continue
+                    fsdp_osd_state[unflat_param_name][state_name] = value.cpu()
+
+    return fsdp_osd_state
+
+
+@torch.no_grad()
+def _optim_state_dict(
+    model: nn.Module,
+    optim: torch.optim.Optimizer,
+    optim_state_dict: Dict[str, Any],
+    optim_input: Optional[
+        Union[
+            List[Dict[str, Any]],
+            Iterable[nn.Parameter],
+        ]
+    ],
+    rank0_only: bool,
+    shard_state: bool,
+    group: Optional[dist.ProcessGroup],
+    using_optim_input: bool,
+    use_orig_params: bool = False,
+    cpu_offload: bool = True,
+) -> Dict[str, Any]:
+    """
+    Consolidates the optimizer state and returns it as a :class:`dict`
+    following the convention of :meth:`torch.optim.Optimizer.state_dict`,
+    i.e. with keys ``"state"`` and ``"param_groups"``.
+    The flat parameters in ``FSDP`` modules contained in ``model`` are mapped
+    back to their unflattened parameters.
+
+    Parameter keys are not well-defined. For a regular optimizer, the optimizer
+    state_dict contains a mapping from parameter IDs to parameter states.
+    Parameter IDs are the order of parameters in ``optim.param_groups()`` across
+    all the groups. This API also allows user to pass ``optim_input`` for the
+    mapping between parameters and parameter IDs. Using ``optim_input`` is being
+    deprecated.
+
+    If the optimizer is a ``NamedOptimizer``, the optimizer state_dict does not
+    contain parameter IDs mapping but a mapping from parameter FQNs to parameter
+    states. This API finds the mapping from FQNs to parameters if the optimizer
+    is a ``NamedOptimizer``.
+
+    If ``use_orig_params`` is True, each rank will have all FSDP-managed
+    parameters but some of these parameters may be empty due to the sharding.
+    For a regular optim.Optimizer, states for those empty parameters will
+    not be initialized. So, when aggregating the FQNs across ranks, no assert
+    will be raised on a rank even if it does not have all the states -- it is
+    valid and FSDP knows how to aggregate them. However, FSDP has to ignore
+    handling those parameters that are not managed by FSDP and do not exist on
+    the local rank -- those are managed by other parallelisms and FSDP does not
+    know how to handle/aggregate them.
+
+    Args:
+        model (nn.Module): Root module (which may or may not be a
+            :class:`FullyShardedDataParallel` instance) whose parameters
+            were passed into the optimizer ``optim``.
+        optim (torch.optim.Optimizer): Optimizer for ``model`` 's
+            parameters.
+        rank0_only (bool): If ``True``, saves the populated :class:`dict`
+            only on rank 0; if ``False``, saves it on all ranks. (Default:
+            ``True``)
+        shard_state (bool): If ``True``, shard and distribute all
+            non-zero-dimension states.
+
+    Returns:
+        Dict[str, Any]: A :class:`dict` containing the optimizer state for
+        ``model`` 's original unflattened parameters and including keys
+        "state" and "param_groups" following the convention of
+        :meth:`torch.optim.Optimizer.state_dict`. If ``rank0_only=False``,
+        then nonzero ranks return an empty :class:`dict`.
+    """
+    SimpleProfiler.reset()
+    cm = ExitStack()
+    cm.enter_context(SimpleProfiler.profile(SimpleProfiler.Type.ALL))
+    _reset_flat_param_grad_info_if_needed(traversal_utils._get_fsdp_handles(model))
+    to_save = not rank0_only or dist.get_rank(group) == 0 or shard_state
+
+    with SimpleProfiler.profile("preprocessing"):
+        param_to_fqns = _get_param_to_fqns(model)
+        flat_param_to_fqn = _get_flat_param_to_fqn(model)
+        is_named_optimizer = _is_named_optimizer(optim_state_dict)
+
+        param_key_to_param = cast(
+            Dict[Union[int, str], nn.Parameter],
+            (
+                _get_param_id_to_param_from_optim_input(model, optim_input)
+                if using_optim_input
+                else _get_param_key_to_param(
+                    optim, model, is_named_optimizer, param_to_fqns, flat_param_to_fqn
+                )
+            ),
+        )
+        fqn_to_fsdp_param_info = _get_fqn_to_fsdp_param_info(model)
+
+    with SimpleProfiler.profile("preprocessing_with_comm"):
+        (
+            all_optim_state_keys,
+            optim_state_key_to_param_key,
+        ) = _map_param_key_to_optim_keys(
+            optim_state_dict,
+            group,
+            param_key_to_param,
+            param_to_fqns,
+            fqn_to_fsdp_param_info,
+            merge_keys=use_orig_params,
+        )
+
+    with SimpleProfiler.profile("state_converting"):
+        convert_fn = (
+            _convert_state_with_orig_params
+            if use_orig_params
+            else _convert_state_with_flat_params
+        )
+        fsdp_osd_state = convert_fn(
+            all_optim_state_keys,
+            optim_state_key_to_param_key,
+            fqn_to_fsdp_param_info,
+            optim_state_dict["state"],
+            to_save,
+            shard_state,
+            cpu_offload,
+        )
+
+    # At this point, communication is complete and ranks can return early if nothing
+    # will be saved on that rank.
+    if not to_save:
+        return {}
+
+    fsdp_osd: Dict[str, Any] = {"state": fsdp_osd_state}
+
+    flat_param_fqns = set(flat_param_to_fqn.values())
+    for key, value in optim_state_dict["state"].items():
+        if key in fsdp_osd_state:
+            continue
+        if key in flat_param_fqns:
+            continue
+        if key in param_key_to_param:
+            continue
+        # This key is not recognized by FSDP. It may be a user-defined state
+        # or some parameters state that FSDP is unable to map from
+        # ``optim.param_groups``.
+        warnings.warn(
+            f"Found a optim state, {key}, that FSDP cannot process. FSDP "
+            "will directly copy everything to the returned state_dict. In "
+            "most cases, this is a user-defined state that is not "
+            "associated with any particular parameter. Another possible "
+            "case is this state is managed by TorchRec. Otherwise, there may "
+            " be a mismatched assumption of optim_state_dict of this mode."
+        )
+        fsdp_osd_state[key] = value
+
+    if "param_groups" in optim_state_dict:
+        fsdp_osd["param_groups"] = _unflatten_param_groups(
+            optim_state_dict, param_key_to_param, param_to_fqns
+        )
+
+    cm.close()
+    SimpleProfiler.dump_and_reset("FSDP _optim_state_dict() profiling: ")
+
+    return fsdp_osd
+
+
+def _get_fqn_to_fsdp_param_info(model: nn.Module) -> Dict[str, FSDPParamInfo]:
+    """
+    Construct the mapping from a param's fqn to its corresponding ``FSDPParamInfo``
+    if the param is managed by FSDP. Shared parameters, or original parameters that
+    are shared across multiple nn.Modules, are required to belong to one and only
+    one FSDP instance and thus correspond to one ``FlatParameter``. Within the one
+    ``FlatParameter``, ``FlatParameter._fqns`` only stores the first FQN of a shared
+    parameter. Thus, the keys in the mapping are guaranteed to map to unique parameters.
+    """
+
+    def module_fn(module, prefix, tree_level, fqn_to_param_info):
+        fsdp_state = _get_module_fsdp_state_if_fully_sharded_module(module)
+        if fsdp_state is None:
+            return
+        _lazy_init(fsdp_state, module)
+        handle = _module_handle(fsdp_state, module)
+        if not handle:
+            return
+        flat_param = handle.flat_param
+        fsdp_param_info = FSDPParamInfo(fsdp_state, handle, {}, [])
+        # NOTE: `idx` indexes into the data structures *without* padding
+        # elements
+        for idx, local_fqn in enumerate(flat_param._fqns):
+            fqn = clean_tensor_name(prefix + local_fqn)
+            if fqn in fqn_to_param_info:
+                assert fqn_to_param_info[fqn].handle.flat_param is flat_param, fqn
+            fqn_to_param_info[fqn] = fsdp_param_info
+            fsdp_param_info.param_indices[fqn] = idx
+            if flat_param._params is not None:
+                fsdp_param_info.param_requires_grad.append(
+                    flat_param._params[idx].requires_grad
+                )
+
+    def return_fn(fqn_to_param_info):
+        return fqn_to_param_info
+
+    fqn_to_param_info: Dict[str, FSDPParamInfo] = {}
+    # FlatParameter._fqns stores the local fqn, starting from the root of the
+    # FSDP. Using _apply_to_modules() with model (may not be the FSDP root
+    # module) allows us to construct the global fqn.
+    return _apply_to_modules(
+        model,
+        module_fn,
+        return_fn,
+        [fqn for fqn, _ in _named_parameters_with_duplicates(model)],
+        fqn_to_param_info,
+    )
+
+
+@no_type_check
+def _set_optim_use_dtensor(
+    fsdp_state: _FSDPState,
+    state_dict_settings: StateDictSettings,
+) -> None:
+    # If device_mesh is passed in when initalizing FSDP, we automatically turn the
+    # _use_dtensor flag to be true for ShardedOptimStateDictConfig() if state_dict_type
+    # has to be set to SHARDED_STATE_DICT.
+    if getattr(fsdp_state, "_device_mesh", None):
+        state_dict_type = state_dict_settings.state_dict_type
+        if state_dict_type == StateDictType.LOCAL_STATE_DICT:
+            raise RuntimeError(
+                "Found state_dict_type LOCAL_STATE_DICT.",
+                "DeviceMesh is not compatible with LOCAL_STATE_DICT.",
+                "Please set state_dict_type to SHARDED_STATE_DICT to get DTensor state_dict.",
+            )
+        else:
+            state_dict_settings.optim_state_dict_config._use_dtensor = True

janus/lib/python3.10/site-packages/torch/distributed/fsdp/_traversal_utils.py

@@ -0,0 +1,113 @@
+"""
+NOTE: This file must be imported like
+``import torch.distributed.fsdp._traversal_utils`` and not like
+``from torch.distirbuted.fsdp._traversal_utils import ...`` to avoid circular
+imports. For brevity, we may import the file as ``traversal_utils``.
+"""
+
+import collections
+from typing import Deque, List, Set, Tuple
+
+import torch.nn as nn
+from torch.distributed._composable.contract import _get_registry
+from torch.distributed.fsdp._common_utils import _FSDPState, _get_module_fsdp_state
+
+
+"""
+[Note: FSDP State Traversal]
+For the wrapper code path, ``_FSDPState`` is the ``FullyShardedDataParallel``
+module wrapping a fully sharded module, and for the non-wrapper code path,
+``_FSDPState`` is an object that gets embedded on a fully sharded module.
+See [Note: Fully Sharded Module] for the definition.
+
+There are three common traversal idioms: Given a root module,
+- ``_get_fsdp_states()`` returns all ``_FSDPState`` s in the tree.
+- ``get_fsdp_root_states()`` returns all local root ``_FSDPState`` s in the
+tree (i.e. those with ``_is_root == True``).
+- ``_get_fsdp_handles()``returns all ``FlatParamHandle`` s in the tree.
+
+All of these methods must take in the root module (i.e. an ``nn.Module``) and
+not a general ``_FSDPState`` because ``_FSDPState`` does not support a graph
+traversal, whereas ``nn.Module`` has ``nn.Module.modules()`` for traversal.
+"""
+
+
+def _composable(module: nn.Module) -> bool:
+    """
+    Returns if ``module`` can compose with ``fully_shard``.
+    """
+    # TODO: Add any other composable APIs that are mutually exclusive.
+    registry = _get_registry(module)
+    if registry is None:
+        return True
+    return "replicate" not in registry
+
+
+# TODO (awgu): We may be able to remove this function if we retired the
+# `use_orig_params=False` code path since so far we only need the module for
+# `FlatParameter` registration, which is not needed for `use_orig_params=True`.
+def _get_fsdp_states_with_modules(
+    module: nn.Module,
+) -> Tuple[List[_FSDPState], List[nn.Module]]:
+    """
+    Returns a tuple containing:
+    1. A list of the ``_FSDPState`` instances in the module tree rooted at
+    ``module`` without any duplicates and following the ``module.modules()``
+    traversal order (which is assumed to be depth-first).
+    2. A corresponding list of the modules owning the states in the first list.
+
+    For the wrapper code path, both returned lists are the same, each
+    containing all ``FullyShardedDataParallel`` instances. For the composable
+    code path, this returns a list of all composable state instances and a list
+    of the corresponding fully sharded modules. See [Note: Fully Sharded
+    Module].
+
+    NOTE: The traversal does not proceed into any module annotated by an
+    incompatible API (e.g. ``replicate``).
+    """
+    fsdp_states: List[_FSDPState] = []
+    fsdp_modules: List[nn.Module] = []
+    # Track the visited FSDP states since multiple modules may share the same
+    # one and we want to return a de-duplicated list
+    visited_fsdp_states: Set[_FSDPState] = set()
+    # Track the visited modules in case of shared modules, which implies the
+    # module graph is no longer a tree
+    visited_modules: Set[nn.Module] = set()
+
+    # Perform depth-first search from `module` to ensure that we do not
+    # traverse into an incompatible API's subtree (use DFS instead of BFS to
+    # match `.modules()` order)
+    deque: Deque[nn.Module] = collections.deque([module])
+    while deque:
+        submodule = deque.popleft()
+        visited_modules.add(submodule)
+        if not _composable(submodule):
+            continue
+        for child_module in reversed(list(submodule.children())):
+            if child_module not in visited_modules:
+                deque.appendleft(child_module)
+        optional_state = _get_module_fsdp_state(submodule)
+        if optional_state is not None and optional_state not in visited_fsdp_states:
+            visited_fsdp_states.add(optional_state)
+            fsdp_states.append(optional_state)
+            fsdp_modules.append(submodule)
+    return fsdp_states, fsdp_modules
+
+
+def _get_fsdp_states(module: nn.Module) -> List[_FSDPState]:
+    """See :func:`_get_fsdp_states_with_modules`."""
+    fsdp_states, _ = _get_fsdp_states_with_modules(module)
+    return fsdp_states
+
+
+def _get_fsdp_handles(module: nn.Module) -> List:
+    """
+    Returns all ``FlatParamHandle`` s in the module tree rooted at ``module``
+    following the rules in :func:`_get_fsdp_state`.
+    """
+    handles = [
+        fsdp_state._handle
+        for fsdp_state in _get_fsdp_states(module)
+        if fsdp_state._handle is not None
+    ]
+    return handles

janus/lib/python3.10/site-packages/torch/distributed/fsdp/_unshard_param_utils.py

@@ -0,0 +1,336 @@
+# mypy: allow-untyped-defs
+import contextlib
+import warnings
+from typing import cast, Generator
+
+import torch
+import torch.distributed.fsdp._traversal_utils as traversal_utils
+import torch.nn as nn
+from torch.distributed.fsdp._common_utils import (
+    _FSDPState,
+    _get_module_fsdp_state,
+    _has_fsdp_params,
+    _module_handle,
+    HandleTrainingState,
+    TrainingState,
+)
+from torch.distributed.fsdp._runtime_utils import (
+    _lazy_init,
+    _reset_flat_param_grad_info_if_needed,
+    _reshard,
+    _reshard_grads,
+    _unshard,
+    _unshard_grads,
+)
+from torch.distributed.utils import _p_assert
+
+from ._flat_param import FlatParamHandle
+
+
+FLAT_PARAM = "_flat_param"
+
+
+@torch.no_grad()
+def _writeback_to_local_shard(
+    handle: FlatParamHandle,
+    writeback_grad: bool,
+):
+    """
+    For the handle, writes back the this rank's shard of the unsharded
+    flattened parameter to the sharded flattened parameter. If
+    ``writeback_grad=True``, then writes back to the sharded gradient as
+    well.
+
+    Precondition: The handle's ``FlatParameter`` 's data points to the
+    padded unsharded flattened parameter.
+    """
+
+    def _get_shard(flat_param_or_grad: torch.Tensor) -> torch.Tensor:
+        if handle.uses_sharded_strategy:
+            # For sharded strategies, get the *unpadded* shard instead of
+            # the *padded* shard to persist user changes to the padding
+            # (though FSDP does not explicitly support this)
+            shard, _ = FlatParamHandle._get_unpadded_shard(
+                flat_param_or_grad,
+                handle.rank,
+                handle.world_size,
+            )
+            return shard
+        # For `NO_SHARD`, the `flat_param` or its gradient may be modified,
+        # so we write it back directly
+        return flat_param_or_grad
+
+    param_shard = _get_shard(handle.flat_param)
+    handle.flat_param._local_shard[: param_shard.numel()].copy_(param_shard)  # type: ignore[attr-defined]
+    if writeback_grad:
+        existing_grad = handle.sharded_grad
+        if existing_grad is not None:
+            assert handle.flat_param.grad is not None
+            grad_shard = _get_shard(handle.flat_param.grad)
+            existing_grad[: grad_shard.numel()].copy_(grad_shard)
+
+
+def _deregister_flat_param(state: _FSDPState, module: nn.Module) -> None:
+    """
+    De-registers the flattened parameter from the wrapped module, hiding it
+    from ``nn.Module`` methods.
+
+    We do not use ``del`` because we want ``FLAT_PARAM`` to always be an
+    attribute but dynamically change whether it is visible to ``nn.Module``
+    methods.
+    """
+    if _has_fsdp_params(state, module):
+        # TODO: figure out the case for the composable APIs.
+        cast(nn.Module, module.module)._parameters.pop(FLAT_PARAM, None)
+
+
+def _register_flat_param(state: _FSDPState, module: nn.Module) -> None:
+    """
+    Registers the flattened parameter to the wrapped module, making it
+    visible to ``nn.Module`` methods.
+
+    We do not use :meth:`nn.Module.register_parameter` because we want
+    ``FLAT_PARAM`` to always be an attribute but dynamically change whether
+    it is visible to ``nn.Module`` methods.
+    """
+    handle = _module_handle(state, module)
+    if _has_fsdp_params(state, module):
+        # TODO: figure out the case for the composable APIs.
+        cast(nn.Module, module.module)._parameters[FLAT_PARAM] = handle.flat_param
+
+
+@contextlib.contextmanager
+def _unflatten_as_params(state: _FSDPState, module: nn.Module) -> Generator:
+    """
+    Assumes that the flattened parameter is unsharded. When in the context,
+    de-registers the flattened parameter and unflattens the original
+    parameters as ``nn.Parameter`` views into the flattened parameter.
+    After the context, re-registers the flattened parameter and restores
+    the original parameters as ``Tensor`` views into the flattened
+    parameter.
+    """
+    handle = _module_handle(state, module)
+    if not handle:
+        yield
+    else:
+        _deregister_flat_param(state, module)
+        try:
+            with handle.unflatten_as_params():
+                yield
+        finally:
+            if not handle._use_orig_params:
+                _register_flat_param(state, module)
+
+
+def _validate_unshard_params_args(
+    state: _FSDPState,
+    writeback: bool,
+    rank0_only: bool,
+    offload_to_cpu: bool,
+    with_grads: bool,
+) -> None:
+    if with_grads and (offload_to_cpu or not state._use_orig_params):
+        raise NotImplementedError(
+            f"with_grads={with_grads}, "
+            f"use_orig_params={state._use_orig_params}, "
+            f"offload_to_cpu={offload_to_cpu} "
+            f"is not supported yet"
+        )
+    if offload_to_cpu and state._handle and (not state._handle.uses_sharded_strategy):
+        raise NotImplementedError(
+            "offload_to_cpu=True and NO_SHARD is not supported yet"
+        )
+    if writeback and rank0_only:
+        # TODO: Rank 0 can broadcast the `FlatParameter` to allow all ranks to
+        # persist the changes.
+        raise NotImplementedError(
+            "writeback=True and rank0_only=True is not supported yet"
+        )
+    if offload_to_cpu and not rank0_only:
+        warnings.warn(
+            "offload_to_cpu=True and rank0_only=False may result in the"
+            "unsharded parameters being redundantly copied to CPU memory for "
+            "GPUs sharing the same CPU memory, which risks CPU OOM. We "
+            "recommend using offload_to_cpu=True with rank0_only=True."
+        )
+
+
+@contextlib.contextmanager
+def _unshard_fsdp_state_params(
+    module: nn.Module,
+    state: _FSDPState,
+    writeback: bool,
+    rank0_only: bool,
+    offload_to_cpu: bool,
+    with_grads: bool,
+):
+    """
+    This unshards the parameters for a single FSDP state ``state`` that
+    corresponds to ``module``.
+    """
+    _validate_unshard_params_args(
+        state, writeback, rank0_only, offload_to_cpu, with_grads
+    )
+    state._device_handle.synchronize()
+    # If handles are shared by other module(s), the handle may be already unsharded.
+    maybe_handle = _module_handle(state, module)
+    handle = None
+    if (
+        maybe_handle
+        and maybe_handle._training_state != HandleTrainingState.SUMMON_FULL_PARAMS
+    ):
+        handle = maybe_handle
+    if not handle:
+        yield
+        return
+
+    assert (
+        handle._training_state == HandleTrainingState.IDLE
+    ), f"Expects the handle training to be IDLE but got {handle._training_state}"
+
+    handle._training_state = HandleTrainingState.SUMMON_FULL_PARAMS
+
+    _reset_flat_param_grad_info_if_needed(handle)
+    free_unsharded_flat_param = handle.needs_unshard()
+    # No need to call `wait_stream()` since we unshard in the computation
+    # stream directly
+    computation_stream = state._device_handle.current_stream()
+    _unshard(state, handle, computation_stream, computation_stream)
+    if with_grads:
+        _unshard_grads(handle)
+
+    if rank0_only and state.rank != 0:
+        # Free the unsharded flattened parameter early
+        _reshard(state, handle, free_unsharded_flat_param)
+        if with_grads:
+            _reshard_grads(handle)
+        try:
+            yield
+        finally:
+            handle._training_state = HandleTrainingState.IDLE
+    else:
+        # Unflatten the unsharded flattened parameters
+        with contextlib.ExitStack() as stack:
+            # Invariant: rank == 0 or !rank0_only
+            if offload_to_cpu and handle.uses_sharded_strategy:
+                stack.enter_context(handle.to_cpu())
+                # NOTE: Since PyTorch enforces that a parameter and its
+                # gradients need to match metadata (e.g. device), we must
+                # move gradients to CPU *after* we move parameters.
+            # NOTE: This assumes 1 `FlatParameter`
+            if not state._use_orig_params:
+                stack.enter_context(_unflatten_as_params(state, module))
+            try:
+                yield
+            finally:
+                stack.close()
+                if writeback:
+                    _writeback_to_local_shard(handle, with_grads)
+                _reshard(state, handle, free_unsharded_flat_param)
+                if with_grads:
+                    _reshard_grads(handle)
+                handle._training_state = HandleTrainingState.IDLE
+
+
+@contextlib.contextmanager
+def _unshard_params_for_summon(
+    module: nn.Module,
+    state: _FSDPState,
+    writeback: bool,
+    rank0_only: bool,
+    offload_to_cpu: bool,
+    with_grads: bool,
+):
+    _validate_unshard_params_args(
+        state, writeback, rank0_only, offload_to_cpu, with_grads
+    )
+    _lazy_init(state, module)
+    if state.training_state == TrainingState.FORWARD_BACKWARD:
+        raise AssertionError(
+            "Cannot manually unshard parameters during forward/backward"
+        )
+    elif state.training_state == TrainingState.SUMMON_FULL_PARAMS:
+        raise AssertionError(
+            "Cannot manually unshard parameters when already unsharding parameters"
+        )
+    with _unshard_fsdp_state_params(
+        module=module,
+        state=state,
+        writeback=writeback,
+        rank0_only=rank0_only,
+        offload_to_cpu=offload_to_cpu,
+        with_grads=with_grads,
+    ):
+        try:
+            state.training_state = TrainingState.SUMMON_FULL_PARAMS
+            yield
+        finally:
+            state.training_state = TrainingState.IDLE
+
+
+@contextlib.contextmanager
+def _unshard_params(
+    module: nn.Module,
+    recurse: bool,
+    writeback: bool,
+    rank0_only: bool,
+    offload_to_cpu: bool,
+    with_grads: bool,
+):
+    """
+    This unshards FSDP-managed parameters for all modules with FSDP applied in
+    the module tree rooted at ``module``.
+    """
+    if not recurse:
+        optional_state = _get_module_fsdp_state(module)
+        if optional_state is None:
+            with contextlib.nullcontext():
+                yield
+            return
+        states_and_modules = ([optional_state], [module])
+    else:
+        states_and_modules = traversal_utils._get_fsdp_states_with_modules(module)
+    with contextlib.ExitStack() as stack:
+        for state, module in zip(*states_and_modules):
+            stack.enter_context(
+                _unshard_params_for_summon(
+                    module=module,
+                    state=state,
+                    writeback=writeback,
+                    rank0_only=rank0_only,
+                    offload_to_cpu=offload_to_cpu,
+                    with_grads=with_grads,
+                )
+            )
+        yield
+
+
+def _deregister_orig_params(state: _FSDPState, module: nn.Module) -> None:
+    """
+    Deregisters the original parameters; registers the ``FlatParameter``.
+    """
+    handle = _module_handle(state, module)
+    if not handle:
+        return
+    _p_assert(
+        handle._use_orig_params,
+        f"Inconsistent `_use_orig_params` -- FSDP: {state._use_orig_params} "
+        f"handle: {handle._use_orig_params}",
+    )
+    handle._deregister_orig_params()
+    _register_flat_param(state, module)
+
+
+def _register_orig_params(state: _FSDPState, module: nn.Module) -> None:
+    """
+    Deregisters the ``FlatParameter``; registers the original parameters.
+    """
+    handle = _module_handle(state, module)
+    if not handle:
+        return
+    _deregister_flat_param(state, module)
+    if handle.is_sharded(handle.flat_param):
+        handle._use_sharded_views()
+        handle._use_sharded_grad_views()
+    else:
+        handle._use_unsharded_views(as_params=True)

janus/lib/python3.10/site-packages/torch/distributed/fsdp/_wrap_utils.py

@@ -0,0 +1,262 @@
+# mypy: allow-untyped-defs
+import collections
+import functools
+import inspect
+import warnings
+from functools import partial
+from typing import Any, Callable, Dict, List, Set, Tuple, Type, Union
+
+import torch.nn as nn
+from torch.distributed.fsdp._common_utils import (
+    _get_module_fsdp_state,
+    _override_module_mixed_precision,
+)
+from torch.distributed.fsdp.wrap import (
+    _construct_wrap_fn,
+    _or_policy,
+    _Policy,
+    _post_order_apply,
+    _recursive_wrap,
+    _run_mixed_precision_override_policy,
+    _wrap_module_cls_individually,
+)
+
+
+def _auto_wrap(
+    root_module: nn.Module,
+    policy: Union[Callable, _Policy],
+    ignored_modules: Set[nn.Module],
+    ignored_params: Set[nn.Parameter],
+    root_kwargs: Dict[str, Any],
+    fsdp_fn: Callable,  # e.g. `FullyShardedDataParallel` or `fully_shard`
+):
+    """
+    Auto wraps modules in ``root_module`` 's tree according to ``policy``
+    following a post-order traversal.
+
+    Precondition: ``root_kwargs`` should contain all arguments except
+    ``module``. This function accepts the kwargs dict directly since it gets
+    forwarded into the post-order traversal function.
+    """
+    mixed_precision = root_kwargs["mixed_precision"]
+    is_wrapper = inspect.isclass(fsdp_fn)
+    # TODO: We may relax this no-nested-wrapping constraint to support manual
+    # wrapping followed by auto wrapping.
+    _check_nested_wrapping(root_module)
+
+    if isinstance(policy, _Policy):
+        root_kwargs["auto_wrap_policy" if is_wrapper else "policy"] = None
+        target_module_to_kwargs = policy._run_policy(
+            root_module, ignored_modules, root_kwargs
+        )
+        if mixed_precision is not None:
+            target_module_to_kwargs = _run_mixed_precision_override_policy(
+                root_module,
+                mixed_precision._module_classes_to_ignore,
+                ignored_modules,
+                root_kwargs,
+                target_module_to_kwargs,
+            )
+            overridden_module_classes = _override_module_mixed_precision(
+                root_module, mixed_precision._module_classes_to_ignore
+            )
+            _warn_on_overridden_mixed_precision(overridden_module_classes)
+        use_orig_params = root_kwargs.get("use_orig_params", False)
+        _validate_frozen_params(
+            root_module,
+            set(target_module_to_kwargs.keys()),
+            ignored_params,
+            use_orig_params,
+        )
+        wrap_fn = _construct_wrap_fn(root_module, target_module_to_kwargs, fsdp_fn)
+        _post_order_apply(root_module, wrap_fn)
+        return
+
+    recursive_wrap_kwargs = {
+        "module": root_module,
+        "auto_wrap_policy": policy,
+        "wrapper_cls": fsdp_fn,
+        "ignored_modules": ignored_modules,
+        "ignored_params": ignored_params,
+        "only_wrap_children": True,
+    }
+    if mixed_precision is not None:
+        # Wrap modules of the ignored types separately and register forward
+        # hooks to cast to fp32 and back to the original dtype, respectively
+        overridden_module_classes = _override_module_mixed_precision(
+            root_module, mixed_precision._module_classes_to_ignore
+        )
+        policy = functools.partial(
+            _or_policy,
+            policies=[
+                policy,
+                partial(
+                    _wrap_module_cls_individually,
+                    module_classes=mixed_precision._module_classes_to_ignore,
+                ),
+            ],
+        )
+        recursive_wrap_kwargs["auto_wrap_policy"] = policy
+        _warn_on_overridden_mixed_precision(overridden_module_classes)
+    _recursive_wrap(**recursive_wrap_kwargs, **root_kwargs)  # type: ignore[arg-type]
+
+
+def _check_nested_wrapping(root_module: nn.Module):
+    for module_name, module in root_module.named_modules():
+        if _get_module_fsdp_state(module) is not None:
+            raise ValueError(
+                "FSDP auto wrapping requires modules to not already have "
+                f"FSDP applied but found {module_name} in\n{root_module}"
+            )
+
+
+def _warn_on_overridden_mixed_precision(
+    overridden_module_classes: Set[Type[nn.Module]],
+):
+    if len(overridden_module_classes) == 0:
+        return
+    warnings.warn(
+        "Both mixed precision and an auto_wrap_policy were specified to FSDP, "
+        f"where the wrapped module has submodules of type:\n{overridden_module_classes}\n"
+        "These modules will be wrapped as separate FSDP instacnes with mixed "
+        "precision disabled."
+    )
+
+
+def _validate_frozen_params(
+    root_module: nn.Module,
+    modules_to_wrap: Set[nn.Module],
+    ignored_params: Set[nn.Parameter],
+    use_orig_params: bool,
+):
+    """
+    This checks that, given ``modules_to_wrap``, each module would manage
+    parameters that are uniformly frozen or non-frozen. This uniformity
+    requirement is strict for ``use_orig_params=False`` (hard error) and highly
+    recommended for ``use_orig_params=True`` (user warning).
+    """
+    post_order_named_modules = _get_post_order_named_modules(root_module)
+    visited_modules: Set[nn.Module] = set()
+    for module_name, module in post_order_named_modules:
+        if module in modules_to_wrap:
+            param_to_fqn = _get_managed_param_to_fqn(
+                module, ignored_params, visited_modules, module_name
+            )
+            frozen_param_fqns: List[str] = []
+            frozen_param_numel = 0
+            nonfrozen_param_fqns: List[str] = []
+            nonfrozen_param_numel = 0
+            for param, fqn in param_to_fqn.items():
+                if param.requires_grad:
+                    nonfrozen_param_fqns.append(fqn)
+                    nonfrozen_param_numel += param.numel()
+                else:
+                    frozen_param_fqns.append(fqn)
+                    frozen_param_numel += param.numel()
+            if len(frozen_param_fqns) > 0 and len(nonfrozen_param_fqns) > 0:
+                msg = f"{module_name} has both parameters with requires_grad=True and False."
+                if use_orig_params:
+                    total_param_numel = frozen_param_numel + nonfrozen_param_numel
+                    msg += (
+                        " We do not recommend wrapping such modules since "
+                        "the gradient memory usage will be higher than expected "
+                        f"({total_param_numel} numel instead of {nonfrozen_param_numel} numel "
+                        "before sharding via reduce-scatter). "
+                    )
+                else:
+                    msg += " FSDP does not support wrapping such modules when use_orig_params=False. "
+                msg += "If possible, wrap the frozen parameters with FSDP separately.\n"
+                msg += (
+                    f"The following parameters have requires_grad=True:\n{nonfrozen_param_fqns}\n"
+                    f"The following parameters have requires_grad=False:\n{frozen_param_fqns}"
+                )
+                if use_orig_params:
+                    warnings.warn(msg)
+                else:
+                    raise ValueError(msg)
+
+
+def _get_post_order_named_modules(
+    root_module: nn.Module,
+) -> List[Tuple[str, nn.Module]]:
+    """
+    This returns the named modules following a post-order traversal, which is a
+    valid reverse topological sort. We achieve this using the reverse of a
+    stack-based DFS order instead of reversing ``root_module.named_modules()``
+    since the former gives the modules in registration order at each level in
+    the module tree (as opposed to the reverse), which allows us to error/warn
+    on the first registered module that violates the condition.
+
+    For example, consider the following module structure:
+        M(
+          S1(),
+          S2(
+            SS1(),
+            SS2(),
+          ),
+          S3(),
+        )
+    The reverse DFS order is [S1, SS1, SS2, S2, S3, M], while the reverse
+    ``named_modules()`` order is [S3, SS2, SS1, S2, S1, M].
+    """
+    visited_modules = {root_module}
+    stack = [("", root_module)]
+    # Append and reverse at the end for linear-time algorithm
+    reverse_post_order_named_modules: List[Tuple[str, nn.Module]] = []
+    while stack:
+        module_name, module = stack.pop()
+        reverse_post_order_named_modules.append((module_name, module))
+        for child_module_name, child_module in module.named_children():
+            if child_module is None:  # only for overrides of `named_children()`
+                continue
+            if child_module not in visited_modules:
+                visited_modules.add(child_module)
+                if module_name != "":
+                    child_module_name = module_name + "." + child_module_name
+                stack.append((child_module_name, child_module))
+    post_order_named_modules = list(reversed(reverse_post_order_named_modules))
+    return post_order_named_modules
+
+
+def _get_managed_param_to_fqn(
+    module_to_wrap: nn.Module,
+    ignored_params: Set[nn.Parameter],
+    visited_modules: Set[nn.Module],
+    root_prefix: str,
+) -> Dict[nn.Parameter, str]:
+    """
+    This returns a dict that maps managed parameter to its FQN for the given
+    ``module_to_wrap``. The dict's keys are exactly the parameters that would
+    be managed by the module, where this is achieved by calling this function
+    on the modules to wrap in reverse topological order, destructively updating
+    ``visited_modules``, and not traversing into those modules. The FQNs are
+    prefixed from the root (via ``root_prefix``) to be more informative.
+
+    NOTE: This function is meant to be called pre-wrapping and iteratively in
+    reverse topological order to cover the full module tree. This differs from
+    the ``_get_param_to_fqn()`` function meant to be called post-wrapping and
+    on the full module tree in one shot. Given those differences, we do not try
+    to unify the two.
+    """
+    param_to_fqn: Dict[nn.Parameter, str] = {}
+    # Run BFS (or any tree traversal works)
+    queue = collections.deque([(module_to_wrap, root_prefix)])
+    visited_modules.add(module_to_wrap)
+    while queue:
+        module, prefix = queue.popleft()
+        for param_name, param in module.named_parameters(recurse=False):
+            if param not in ignored_params:
+                fqn = param_name if prefix == "" else prefix + "." + param_name
+                param_to_fqn[param] = fqn
+        for child_module_name, child_module in module.named_children():
+            if child_module is None:  # only for overrides of `named_children()`
+                continue
+            if child_module not in visited_modules:
+                visited_modules.add(child_module)
+                child_prefix = (
+                    child_module_name
+                    if prefix == ""
+                    else prefix + "." + child_module_name
+                )
+                queue.append((child_module, child_prefix))
+    return param_to_fqn

janus/lib/python3.10/site-packages/torch/distributed/fsdp/sharded_grad_scaler.py

@@ -0,0 +1,396 @@
+# mypy: allow-untyped-defs
+import logging
+from collections import abc, defaultdict
+from typing import Any, Dict, Iterable, List, Optional, overload, Sequence, Tuple, Union
+
+import torch
+import torch.distributed as dist
+from torch.amp.grad_scaler import _MultiDeviceReplicator, GradScaler, OptState
+from torch.distributed.distributed_c10d import ProcessGroup
+
+
+logger = logging.getLogger(__name__)
+
+
+def _refresh_per_optimizer_state() -> Dict[str, Any]:
+    return {"stage": OptState.READY, "found_inf_per_device": {}}
+
+
+def _is_supported_device(tensor: torch.Tensor) -> bool:
+    return tensor.is_cuda or tensor.device.type in (
+        "xla",
+        "cpu",
+        "hpu",
+        "mtia",
+        torch._C._get_privateuse1_backend_name(),
+    )
+
+
+class _GeneralMultiDeviceReplicator(_MultiDeviceReplicator):
+    """
+    Lazily serves tensor to request device. This class extends
+    _MultiDeviceReplicator to allow support for "cpu" as a device.
+    """
+
+    def __init__(self, master_tensor: torch.Tensor) -> None:
+        assert _is_supported_device(master_tensor)
+        self.master = master_tensor
+        self._per_device_tensors: Dict[torch.device, torch.Tensor] = {}
+
+
+class ShardedGradScaler(GradScaler):
+    """
+    ShardedGradScaler helps perform gradient scaling in a shard aware manner. It extends
+    functionality from GradScaler:
+    * Supports Pytorch DDP and FSDP implementations
+    * Support CPU offloaded tensors (as used in fully sharded data parallel[FSDP])
+    * Supports the custom Mixed Precision loss dtype (fp16, bf16) that FSDP returns
+    * Sync inf/nan for scaled gradient tensors on any torch.device (where tensors are placed) across
+    nodes
+
+    Example::
+
+        # Creates a ShardedGradScaler once at the beginning of training.
+        scaler = ShardedGradScaler()
+
+        for epoch in epochs:
+            for input, target in data:
+                optimizer.zero_grad()
+                output = model(input)
+                loss = loss_fn(output, target)
+
+                # Scales loss.  Calls backward() on scaled loss to create scaled gradients.
+                scaler.scale(loss).backward()
+
+                # scaler.step() first unscales gradients of the optimizer's params.
+                # If gradients don't contain infs/NaNs, optimizer.step() is then called,
+                # otherwise, optimizer.step() is skipped.
+                scaler.step(optimizer)
+
+                # Updates the scale for next iteration.
+                scaler.update()
+
+    See :class:`GradScaler` for explanation of scaling/unscaling and more use cases.
+
+    Args:
+        init_scale (float, optional, default=2.**16):  Initial scale factor.
+        growth_factor (float, optional, default=2.0):  Factor by which the scale is multiplied during
+            :meth:`update` if no inf/NaN gradients occur for ``growth_interval`` consecutive iterations.
+        backoff_factor (float, optional, default=0.5):  Factor by which the scale is multiplied during
+            :meth:`update` if inf/NaN gradients occur in an iteration.
+        growth_interval (int, optional, default=2000):  Number of consecutive iterations without inf/NaN gradients
+            that must occur for the scale to be multiplied by ``growth_factor``.
+        enabled (bool, optional):  If ``False``, disables gradient scaling. :meth:`step` simply
+            invokes the underlying ``optimizer.step()``, and other methods become no-ops.
+            Default: ``True``
+        process_group (ProcessGroup, optional, default=torch.distributed.group.WORLD):
+            process group for sharding
+    """
+
+    def __init__(
+        self,
+        device: str = "cuda",
+        init_scale: float = 2.0**16,
+        backoff_factor: float = 0.5,
+        growth_factor: float = 2.0,
+        growth_interval: int = 2000,
+        enabled: bool = True,
+        process_group: Optional[ProcessGroup] = dist.group.WORLD,
+    ) -> None:
+        super().__init__(
+            device,
+            init_scale=init_scale,
+            backoff_factor=backoff_factor,
+            growth_factor=growth_factor,
+            growth_interval=growth_interval,
+            enabled=enabled,
+        )
+        if self._enabled:
+            self.process_group = process_group
+            self._per_optimizer_states = defaultdict(_refresh_per_optimizer_state)
+
+    @overload
+    def scale(self, outputs: torch.Tensor) -> torch.Tensor:
+        ...
+
+    @overload
+    def scale(self, outputs: List[torch.Tensor]) -> List[torch.Tensor]:
+        ...
+
+    @overload
+    def scale(self, outputs: Tuple[torch.Tensor, ...]) -> Tuple[torch.Tensor, ...]:
+        ...
+
+    @overload
+    def scale(self, outputs: Iterable[torch.Tensor]) -> Iterable[torch.Tensor]:
+        ...
+
+    def scale(
+        self, outputs: Union[torch.Tensor, Iterable[torch.Tensor]]
+    ) -> Union[torch.Tensor, Iterable[torch.Tensor]]:
+        if not self._enabled:
+            return outputs
+
+        if isinstance(outputs, torch.Tensor):
+            assert _is_supported_device(outputs)
+            if self._scale is None:
+                self._lazy_init_scale_growth_tracker(outputs.device)
+            assert self._scale is not None
+            scaled_output = outputs * self._scale.to(
+                device=outputs.device, non_blocking=True
+            )
+            # Here we ensure the return dtype is the same as the outputs dtype.
+            # For the FSDP + Mixed Precision use case, the loss output is in the Mixed Precision
+            # format (fp16, bf16) and so the scaled loss should be of the same dtype.
+            return scaled_output.type(outputs.dtype)
+
+        stash: List[_GeneralMultiDeviceReplicator] = []
+
+        def apply_scale(val: Union[torch.Tensor, Iterable[torch.Tensor]]):
+            if isinstance(val, torch.Tensor):
+                assert _is_supported_device(val)
+                if len(stash) == 0:
+                    if self._scale is None:
+                        self._lazy_init_scale_growth_tracker(val.device)
+                    assert self._scale is not None
+                    stash.append(_GeneralMultiDeviceReplicator(self._scale))
+                scaled_val = val * stash[0].get(val.device)
+                # Here we ensure the return dtype is the same as the outputs dtype.
+                # For the FSDP + Mixed Precision use case, the loss output is in the Mixed Precision
+                # format (fp16, bf16) and so the scaled loss should be of the same dtype.
+                return scaled_val.type(val.dtype)
+            if isinstance(val, abc.Iterable):
+                iterator = map(apply_scale, val)
+                if isinstance(val, (list, tuple)):
+                    return type(val)(iterator)
+                return iterator
+            raise ValueError("outputs must be a Tensor or an iterable of Tensors")
+
+        return apply_scale(outputs)
+
+    def _foreach_non_finite_check_and_unscale_cpu_(
+        self,
+        grads: Sequence[torch.Tensor],
+        found_inf: torch.Tensor,
+        inv_scale: torch.Tensor,
+    ) -> None:
+        if len(grads) == 0:
+            return
+        assert inv_scale.numel() == 1, "inv_scale must be a 1-element tensor."
+        assert found_inf.numel() == 1, "found_inf must be a 1-element tensor."
+
+        for grad in grads:
+            if grad.device.type != "cpu":
+                logger.error(
+                    "tensor device is %s but was expected to be ``cpu``",
+                    grad.device,
+                )
+                raise ValueError(
+                    "Gradients were found on a non-CPU device when"
+                    " expected to be on CPU."
+                )
+            if (
+                torch.isinf(grad).any().item() is True
+                or torch.isnan(grad).any().item() is True
+            ):
+                found_inf.data = torch.tensor([1.0])
+                break
+            else:
+                grad.data *= inv_scale.item()
+
+    def _unscale_grads_(
+        self,
+        optimizer: torch.optim.Optimizer,
+        inv_scale: torch.Tensor,
+        found_inf: torch.Tensor,
+        allow_fp16: bool = True,
+    ) -> Dict[torch.device, torch.Tensor]:
+        per_device_inv_scale = _GeneralMultiDeviceReplicator(inv_scale)
+        per_device_found_inf = _GeneralMultiDeviceReplicator(found_inf)
+
+        # To set up _amp_foreach_non_finite_check_and_unscale_, split grads by device and dtype.
+        # There could be thousands of grads, so we'd like to iterate through them just once.
+        # However, we don't know their devices or dtypes in advance.
+
+        # https://stackoverflow.com/questions/5029934/defaultdict-of-defaultdict
+        # Google says mypy struggles with defaultdicts type annotations.
+        per_device_and_dtype_grads = defaultdict(lambda: defaultdict(list))  # type: ignore[var-annotated]
+        with torch.no_grad():
+            for group in optimizer.param_groups:
+                for param in group["params"]:
+                    if param.grad is None:
+                        continue
+                    if (not allow_fp16) and param.grad.dtype == torch.float16:
+                        raise ValueError("Attempting to unscale FP16 gradients.")
+                    if param.grad.is_sparse:
+                        # is_coalesced() == False means the sparse grad has values with duplicate indices.
+                        # coalesce() deduplicates indices and adds all values that have the same index.
+                        # For scaled fp16 values, there's a good chance coalescing will cause overflow,
+                        # so we should check the coalesced _values().
+                        if param.grad.dtype is torch.float16:
+                            # coalesce is not supported in torch.float16
+                            param_grad_fp32 = param.grad.type(torch.float32).coalesce()
+                            param.grad = param_grad_fp32.type(torch.float16)
+                        to_unscale = param.grad._values()
+                    else:
+                        to_unscale = param.grad
+
+                    per_device_and_dtype_grads[to_unscale.device][
+                        to_unscale.dtype
+                    ].append(to_unscale)
+
+            for device, per_dtype_grads in per_device_and_dtype_grads.items():
+                for grads in per_dtype_grads.values():
+                    if grads[0].device.type == "cpu":
+                        self._foreach_non_finite_check_and_unscale_cpu_(
+                            grads,
+                            per_device_found_inf.get(device),
+                            per_device_inv_scale.get(device),
+                        )
+                    else:
+                        torch._amp_foreach_non_finite_check_and_unscale_(
+                            grads,
+                            per_device_found_inf.get(device),
+                            per_device_inv_scale.get(device),
+                        )
+        # There exist contexts (e.g. w/ `use_orig_params=True`) wherein some
+        # ranks may have no (non-zero sized) parameter shards, necessitating the
+        # initialization of `per_device_found_inf._per_device_tensors` here
+        if not per_device_found_inf._per_device_tensors:
+            assert self._scale is not None
+            per_device_found_inf.get(self._scale.device)
+        return per_device_found_inf._per_device_tensors
+
+    def unscale_(self, optimizer: torch.optim.Optimizer) -> None:
+        if not self._enabled:
+            return
+
+        self._check_scale_growth_tracker("unscale_")
+
+        optimizer_state = self._per_optimizer_states[id(optimizer)]
+
+        if optimizer_state["stage"] is OptState.UNSCALED:
+            raise RuntimeError(
+                "unscale_() has already been called on this optimizer since the last update()."
+            )
+        elif optimizer_state["stage"] is OptState.STEPPED:
+            raise RuntimeError("unscale_() is being called after step().")
+
+        # FP32 division can be imprecise for certain compile options, so we carry out the reciprocal in FP64.
+        assert self._scale is not None
+        inv_scale = self._scale.double().reciprocal().float()
+        found_inf = torch.full(
+            (1,), 0.0, dtype=torch.float32, device=self._scale.device
+        )
+
+        optimizer_state["found_inf_per_device"] = self._unscale_grads_(
+            optimizer, inv_scale, found_inf, True
+        )
+        optimizer_state["stage"] = OptState.UNSCALED
+
+        # Synchronize the detected inf across the ranks
+        optimizer_state = self._per_optimizer_states[id(optimizer)]
+        works = []
+        found_inf_on_cpus = []
+        found_inf_on_devices = []
+
+        for found_inf in optimizer_state["found_inf_per_device"].values():
+            if self._device != "cpu" and found_inf.device.type == "cpu":
+                found_inf_on_cpus.append(found_inf)
+                found_inf_on_device = found_inf.to(self._device)
+                found_inf_on_devices.append(found_inf_on_device)
+                works.append(
+                    dist.all_reduce(
+                        found_inf_on_device, async_op=True, group=self.process_group
+                    )
+                )
+            else:
+                works.append(
+                    dist.all_reduce(found_inf, async_op=True, group=self.process_group)
+                )
+        for work in works:
+            work.wait()
+        if found_inf_on_cpus:
+            torch._foreach_copy_(found_inf_on_cpus, found_inf_on_devices)
+
+    def _amp_update_scale_cpu_(self, found_inf: torch.Tensor) -> None:
+        """
+        If found_inf is 1.0 (True), then scale is multiplied by backoff_factor and growth_tracker is set to zero.
+        Otherwise, scale is multiplied by the growth factor when the growth interval is reached.
+        """
+        assert self._scale is not None and self._growth_tracker is not None
+
+        if found_inf.item() >= 1.0:
+            self._scale *= self._backoff_factor
+            self._growth_tracker.fill_(0)
+        else:
+            successful = self._growth_tracker + 1
+            if successful == self._growth_interval:
+                self._scale *= self._growth_factor
+                self._growth_tracker.fill_(0)
+            else:
+                self._growth_tracker = successful
+
+    def update(self, new_scale: Optional[Union[float, torch.Tensor]] = None) -> None:
+        """
+        Updates the scale factor.
+        If any optimizer steps were skipped the scale is multiplied by ``backoff_factor``
+        to reduce it. If ``growth_interval`` unskipped iterations occurred consecutively,
+        the scale is multiplied by ``growth_factor`` to increase it.
+        Passing ``new_scale`` sets the new scale value manually. (``new_scale`` is not
+        used directly, it's used to fill GradScaler's internal scale tensor. So if
+        ``new_scale`` was a tensor, later in-place changes to that tensor will not further
+        affect the scale GradScaler uses internally.)
+        Args:
+            new_scale (float or :class:`torch.Tensor`, optional, default=None):  New scale factor.
+        .. warning::
+            :meth:`update` should only be called at the end of the iteration, after ``scaler.step(optimizer)`` has
+            been invoked for all optimizers used this iteration.
+        """
+
+        if not self._enabled:
+            return
+
+        _scale, _growth_tracker = self._check_scale_growth_tracker("update")  # type: ignore[var-annotated]
+
+        if new_scale is not None:
+            # Accept a new user-defined scale.
+            if isinstance(new_scale, float):
+                self._scale.fill_(new_scale)  # type: ignore[union-attr]
+            else:
+                reason = "new_scale should be a float or a 1-element torch.cuda.FloatTensor or \
+                    torch.FloatTensor with requires_grad=False."
+                assert new_scale.device.type == self._device, reason
+                assert new_scale.numel() == 1, reason
+                assert new_scale.requires_grad is False, reason
+                self._scale.copy_(new_scale)  # type: ignore[union-attr]
+        else:
+            # Consume shared inf/nan data collected from optimizers to update the scale.
+            # If all found_inf tensors are on the same device as self._scale, this operation is asynchronous.
+            found_infs = [
+                found_inf.to(device=_scale.device, non_blocking=True)
+                for state in self._per_optimizer_states.values()
+                for found_inf in state["found_inf_per_device"].values()
+            ]
+
+            assert len(found_infs) > 0, "No inf checks were recorded prior to update."
+
+            found_inf_combined = found_infs[0]
+            if len(found_infs) > 1:
+                for i in range(1, len(found_infs)):
+                    found_inf_combined += found_infs[i]
+
+            if _scale.device.type == "cpu":
+                self._amp_update_scale_cpu_(found_inf_combined)
+            else:
+                torch._amp_update_scale_(
+                    self._scale,  # type: ignore[arg-type]
+                    self._growth_tracker,  # type: ignore[arg-type]
+                    found_inf_combined,
+                    self._growth_factor,  # type: ignore[arg-type]
+                    self._backoff_factor,  # type: ignore[arg-type]
+                    self._growth_interval,  # type: ignore[arg-type]
+                )
+
+        # To prepare for next iteration, clear the data collected from optimizers this iteration.
+        self._per_optimizer_states = defaultdict(_refresh_per_optimizer_state)

janus/lib/python3.10/site-packages/torch/distributed/rpc/__init__.py

@@ -0,0 +1,249 @@
+# mypy: allow-untyped-defs
+import logging
+import os
+import threading
+import warnings
+from datetime import timedelta
+from typing import Generator, Tuple
+from urllib.parse import urlparse
+
+import torch
+import torch.distributed as dist
+
+
+__all__ = ["is_available"]
+
+
+logger = logging.getLogger(__name__)
+
+
+_init_counter = 0
+_init_counter_lock = threading.Lock()
+
+
+def is_available() -> bool:
+    return hasattr(torch._C, "_rpc_init")
+
+
+if is_available() and not torch._C._rpc_init():
+    raise RuntimeError("Failed to initialize torch.distributed.rpc")
+
+
+if is_available():
+    import numbers
+
+    import torch.distributed.autograd as dist_autograd
+    from torch._C._distributed_c10d import Store
+    from torch._C._distributed_rpc import (  # noqa: F401
+        _cleanup_python_rpc_handler,
+        _DEFAULT_INIT_METHOD,
+        _DEFAULT_NUM_WORKER_THREADS,
+        _DEFAULT_RPC_TIMEOUT_SEC,
+        _delete_all_user_and_unforked_owner_rrefs,
+        _destroy_rref_context,
+        _disable_jit_rref_pickle,
+        _disable_server_process_global_profiler,
+        _enable_jit_rref_pickle,
+        _enable_server_process_global_profiler,
+        _get_current_rpc_agent,
+        _invoke_remote_builtin,
+        _invoke_remote_python_udf,
+        _invoke_remote_torchscript,
+        _invoke_rpc_builtin,
+        _invoke_rpc_python_udf,
+        _invoke_rpc_torchscript,
+        _is_current_rpc_agent_set,
+        _reset_current_rpc_agent,
+        _rref_context_get_debug_info,
+        _set_and_start_rpc_agent,
+        _set_profiler_node_id,
+        _set_rpc_timeout,
+        _TensorPipeRpcBackendOptionsBase,
+        _UNSET_RPC_TIMEOUT,
+        enable_gil_profiling,
+        get_rpc_timeout,
+        PyRRef,
+        RemoteProfilerManager,
+        RpcAgent,
+        RpcBackendOptions,
+        TensorPipeAgent,
+        WorkerInfo,
+    )
+
+    from . import api, backend_registry, functions
+    from .api import *  # noqa: F401,F403
+    from .backend_registry import BackendType
+    from .options import TensorPipeRpcBackendOptions  # noqa: F401
+    from .server_process_global_profiler import _server_process_global_profile
+
+    rendezvous_iterator: Generator[Tuple[Store, int, int], None, None]
+
+    __all__ += ["init_rpc", "BackendType", "TensorPipeRpcBackendOptions"]
+    __all__ = __all__ + api.__all__ + backend_registry.__all__  # noqa: PLE0605
+
+    def init_rpc(
+        name,
+        backend=None,
+        rank=-1,
+        world_size=None,
+        rpc_backend_options=None,
+    ):
+        r"""
+        Initializes RPC primitives such as the local RPC agent
+        and distributed autograd, which immediately makes the current
+        process ready to send and receive RPCs.
+
+        Args:
+            name (str): a globally unique name of this node. (e.g.,
+                ``Trainer3``, ``ParameterServer2``, ``Master``, ``Worker1``)
+                Name can only contain number, alphabet, underscore, colon,
+                and/or dash, and must be shorter than 128 characters.
+            backend (BackendType, optional): The type of RPC backend
+                implementation. Supported values is
+                ``BackendType.TENSORPIPE`` (the default).
+                See :ref:`rpc-backends` for more information.
+            rank (int): a globally unique id/rank of this node.
+            world_size (int): The number of workers in the group.
+            rpc_backend_options (RpcBackendOptions, optional): The options
+                passed to the RpcAgent constructor. It must be an agent-specific
+                subclass of :class:`~torch.distributed.rpc.RpcBackendOptions`
+                and contains agent-specific initialization configurations. By
+                default, for all agents, it sets the default timeout to 60
+                seconds and performs the rendezvous with an underlying process
+                group initialized using ``init_method = "env://"``,
+                meaning that environment variables ``MASTER_ADDR`` and
+                ``MASTER_PORT`` need to be set properly. See
+                :ref:`rpc-backends` for more information and find which options
+                are available.
+        """
+        torch._C._log_api_usage_once("torch.distributed.init_rpc")
+        if backend is not None and not isinstance(
+            backend, backend_registry.BackendType
+        ):
+            raise TypeError("Argument backend must be a member of BackendType")
+
+        if rpc_backend_options is not None and not isinstance(
+            rpc_backend_options, RpcBackendOptions
+        ):
+            raise TypeError(
+                "Argument rpc_backend_options must be an instance of RpcBackendOptions"
+            )
+
+        # Try to detect the backend from the options
+        if backend is None and rpc_backend_options is not None:
+            for candidate_backend in BackendType:
+                if isinstance(
+                    rpc_backend_options,
+                    type(
+                        backend_registry.construct_rpc_backend_options(
+                            candidate_backend
+                        )
+                    ),
+                ):
+                    backend = candidate_backend
+                    break
+            else:
+                raise TypeError(
+                    f"Could not infer backend for options {rpc_backend_options}"
+                )
+            # Ignore type error because mypy doesn't handle dynamically generated type objects (#4865)
+            if backend != BackendType.TENSORPIPE:  # type: ignore[attr-defined]
+                logger.warning(
+                    "RPC was initialized with no explicit backend but with options "  # type: ignore[attr-defined]
+                    "corresponding to %(backend)s, hence that backend will be used "
+                    "instead of the default BackendType.TENSORPIPE. To silence this "
+                    "warning pass `backend=%(backend)s` explicitly.",
+                    {"backend": backend},
+                )
+
+        if backend is None:
+            backend = BackendType.TENSORPIPE  # type: ignore[attr-defined]
+
+        if rpc_backend_options is None:
+            # default construct a set of RPC backend options.
+            rpc_backend_options = backend_registry.construct_rpc_backend_options(
+                backend
+            )
+
+        # Create store, performs rendezvous for static RPC group.
+        if not world_size:
+            # If world_size is not set in construction and also not set in environment variables
+            # The store will be created for the dynamic group setting
+            store = dist._create_store_from_options(rpc_backend_options, rank)
+        else:
+            # This rendezvous state sometimes is destroyed before all processes
+            # finishing handshaking. To avoid that issue, we make it global to
+            # keep it alive.
+            global rendezvous_iterator
+            rendezvous_iterator = dist.rendezvous(
+                rpc_backend_options.init_method, rank=rank, world_size=world_size
+            )
+            store, _, _ = next(rendezvous_iterator)
+        # Use same timeout as RPC.
+        store.set_timeout(timedelta(seconds=rpc_backend_options.rpc_timeout))
+
+        # Use a PrefixStore to distinguish multiple invocations.
+        with _init_counter_lock:
+            global _init_counter
+            store = dist.PrefixStore(str(f"rpc_prefix_{_init_counter}"), store)
+            _init_counter += 1
+
+        # Initialize autograd before RPC since _init_rpc_backend guarantees all
+        # processes sync via the store. If we initialize autograd after RPC,
+        # there could be a race where some nodes might have initialized autograd
+        # and others might not have. As a result, a node calling
+        # torch.distributed.autograd.backward() would run into errors since
+        # other nodes might not have been initialized.
+        dist_autograd._init(rank)
+
+        _set_profiler_node_id(rank)
+        # Initialize RPC.
+        _init_rpc_backend(backend, store, name, rank, world_size, rpc_backend_options)
+
+    def _validate_rpc_args(backend, store, name, rank, world_size, rpc_backend_options):
+        type_mapping = {
+            backend: backend_registry.BackendType,
+            store: dist.Store,
+            name: str,
+            rank: numbers.Integral,
+            # world_size can be None for a dynamic group
+            world_size: (numbers.Integral, type(None)),
+            rpc_backend_options: RpcBackendOptions,
+        }
+        for arg, arg_type in type_mapping.items():
+            if not isinstance(arg, arg_type):  # type: ignore[arg-type]
+                raise RuntimeError(
+                    f"Argument {arg} must be of type {arg_type} but got type {type(arg)}"
+                )
+
+    def _init_rpc_backend(
+        backend=BackendType.TENSORPIPE,  # type: ignore[attr-defined]
+        store=None,
+        name=None,
+        rank=-1,
+        world_size=None,
+        rpc_backend_options=None,
+    ):
+        _validate_rpc_args(backend, store, name, rank, world_size, rpc_backend_options)
+
+        if _is_current_rpc_agent_set():
+            raise RuntimeError("RPC is already initialized")
+
+        # Initialize RPC.
+        rpc_agent = backend_registry.init_backend(
+            backend,
+            store=store,
+            name=name,
+            rank=rank,
+            world_size=world_size,
+            rpc_backend_options=rpc_backend_options,
+        )
+
+        api._init_rpc_states(rpc_agent)
+
+    @api._require_initialized
+    def _get_debug_info():
+        info = _rref_context_get_debug_info()
+        info.update(api._get_current_rpc_agent().get_debug_info())
+        info.update(dist_autograd._get_debug_info())
+        return info