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import warnings
import torch
from fla.modules.l2norm import l2norm_bwd, l2norm_fwd
from fla.ops.common.chunk_delta_h import chunk_gated_delta_rule_bwd_dhu, chunk_gated_delta_rule_fwd_h
from fla.ops.common.chunk_o import chunk_bwd_dqkwg, chunk_bwd_dv_local, chunk_fwd_o
from fla.ops.delta_rule.wy_fast import prepare_wy_repr_bwd, prepare_wy_repr_fwd, recompute_w_u_fwd
from fla.utils import autocast_custom_bwd, autocast_custom_fwd, input_guard
def chunk_delta_rule_fwd(
q: torch.Tensor,
k: torch.Tensor,
v: torch.Tensor,
beta: torch.Tensor,
scale: float,
initial_state: torch.Tensor,
output_final_state: bool,
cu_seqlens: torch.LongTensor | None = None,
):
# obtain WY representation. u is actually the new v.
w, u, A = prepare_wy_repr_fwd(
k=k,
v=v,
beta=beta,
cu_seqlens=cu_seqlens,
)
h, v_new, final_state = chunk_gated_delta_rule_fwd_h(
k=k,
w=w,
u=u,
g=None,
initial_state=initial_state,
output_final_state=output_final_state,
cu_seqlens=cu_seqlens,
)
o = chunk_fwd_o(
q=q,
k=k,
v=v_new,
h=h,
g=None,
scale=scale,
cu_seqlens=cu_seqlens,
)
return o, A, final_state
def chunk_delta_rule_bwd(
q: torch.Tensor,
k: torch.Tensor,
v: torch.Tensor,
beta: torch.Tensor,
A: torch.Tensor,
scale: float,
initial_state: torch.Tensor,
do: torch.Tensor,
dht: torch.Tensor,
cu_seqlens: torch.LongTensor | None = None,
):
w, u = recompute_w_u_fwd(
k=k,
v=v,
beta=beta,
A=A,
cu_seqlens=cu_seqlens,
)
h, v_new, _ = chunk_gated_delta_rule_fwd_h(
k=k,
w=w,
u=u,
g=None,
initial_state=initial_state,
output_final_state=False,
cu_seqlens=cu_seqlens,
)
dv = chunk_bwd_dv_local(
q=q,
k=k,
do=do,
g=None,
scale=scale,
cu_seqlens=cu_seqlens,
)
dh, dh0, dv = chunk_gated_delta_rule_bwd_dhu(
q=q,
k=k,
w=w,
g=None,
h0=initial_state,
dht=dht,
do=do,
dv=dv,
scale=scale,
cu_seqlens=cu_seqlens,
)
dq, dk, dw, _ = chunk_bwd_dqkwg(
q=q,
k=k,
v=v_new,
h=h,
w=w,
dv=dv,
do=do,
dh=dh,
g=None,
scale=scale,
cu_seqlens=cu_seqlens,
)
dk2, dv, db = prepare_wy_repr_bwd(
k=k,
v=v,
beta=beta,
A=A,
dw=dw,
du=dv,
cu_seqlens=cu_seqlens,
)
dk.add_(dk2)
return dq, dk, dv, db, dh0
class ChunkDeltaRuleFunction(torch.autograd.Function):
@staticmethod
@input_guard
@autocast_custom_fwd
def forward(
ctx,
q: torch.Tensor,
k: torch.Tensor,
v: torch.Tensor,
beta: torch.Tensor,
scale: float,
initial_state: torch.Tensor,
output_final_state: bool,
use_qk_l2norm_in_kernel: bool = False,
cu_seqlens: torch.LongTensor | None = None,
):
if use_qk_l2norm_in_kernel:
q, q_rstd = l2norm_fwd(q)
k, k_rstd = l2norm_fwd(k)
else:
q_rstd, k_rstd = None, None
o, A, final_state = chunk_delta_rule_fwd(
q=q,
k=k,
v=v,
beta=beta,
scale=scale,
initial_state=initial_state,
output_final_state=output_final_state,
cu_seqlens=cu_seqlens,
)
ctx.save_for_backward(q, q_rstd, k, k_rstd, v, beta, A, initial_state)
ctx.scale = scale
ctx.cu_seqlens = cu_seqlens
ctx.use_qk_l2norm_in_kernel = use_qk_l2norm_in_kernel
return o.to(q.dtype), final_state
@staticmethod
@input_guard
@autocast_custom_bwd
def backward(
ctx,
do: torch.Tensor,
dht: torch.Tensor,
):
q, q_rstd, k, k_rstd, v, beta, A, initial_state = ctx.saved_tensors
dq, dk, dv, db, dh0 = chunk_delta_rule_bwd(
q=q,
k=k,
v=v,
beta=beta,
A=A,
scale=ctx.scale,
initial_state=initial_state,
do=do,
dht=dht,
cu_seqlens=ctx.cu_seqlens,
)
if ctx.use_qk_l2norm_in_kernel:
dq = l2norm_bwd(q, q_rstd, dq)
dk = l2norm_bwd(k, k_rstd, dk)
return dq.to(q.dtype), dk.to(k.dtype), dv.to(v.dtype), db.to(beta.dtype), None, dh0, None, None, None, None, None
@torch.compiler.disable
def chunk_delta_rule(
q: torch.Tensor,
k: torch.Tensor,
v: torch.Tensor,
beta: torch.Tensor,
scale: float = None,
initial_state: torch.Tensor = None,
output_final_state: bool = False,
use_qk_l2norm_in_kernel: bool = False,
cu_seqlens: torch.LongTensor | None = None,
head_first: bool = False,
):
r"""
Args:
q (torch.Tensor):
queries of shape `[B, T, H, K]`.
k (torch.Tensor):
keys of shape `[B, T, H, K]`.
v (torch.Tensor):
values of shape `[B, T, H, V]`.
beta (torch.Tensor):
betas of shape `[B, T, H]`.
scale (Optional[float]):
Scale factor for the RetNet attention scores.
If not provided, it will default to `1 / sqrt(K)`. Default: `None`.
initial_state (Optional[torch.Tensor]):
Initial state of shape `[N, H, K, V]` for `N` input sequences.
For equal-length input sequences, `N` equals the batch size `B`.
Default: `None`.
output_final_state (Optional[bool]):
Whether to output the final state of shape `[N, H, K, V]`. Default: `False`.
use_qk_l2norm_in_kernel (Optional[bool]):
Whether to use qk l2norm within the kernel for saving GPU memory.
Default: `False`.
cu_seqlens (torch.LongTensor):
Cumulative sequence lengths of shape `[N+1]` used for variable-length training,
consistent with the FlashAttention API.
head_first (Optional[bool]):
Whether the inputs are in the head-first format. Default: `False`.
This argument has been deprecated.
Returns:
o (torch.Tensor):
Outputs of shape `[B, T, H, V]`.
final_state (torch.Tensor):
Final state of shape `[N, H, K, V]` if `output_final_state=True` else `None`.
Examples::
>>> import torch
>>> import torch.nn.functional as F
>>> from einops import rearrange
>>> from fla.ops.delta_rule import chunk_delta_rule
# inputs with equal lengths
>>> B, T, H, K, V = 4, 2048, 4, 512, 512
>>> q = torch.randn(B, T, H, K, dtype=torch.bfloat16, device='cuda')
>>> k = F.normalize(torch.randn(B, T, H, K, dtype=torch.bfloat16, device='cuda'), p=2, dim=-1)
>>> v = torch.randn(B, T, H, V, dtype=torch.bfloat16, device='cuda')
>>> beta = torch.rand(B, T, H, dtype=torch.bfloat16, device='cuda').sigmoid()
>>> h0 = torch.randn(B, H, K, V, dtype=torch.bfloat16, device='cuda')
>>> o, ht = chunk_delta_rule(
q, k, v, beta,
initial_state=h0,
output_final_state=True
)
# for variable-length inputs, the batch size `B` is expected to be 1 and `cu_seqlens` is required
>>> q, k, v, beta = map(lambda x: rearrange(x, 'b t ... -> 1 (b t) ...'), (q, k, v, beta))
# for a batch with 4 sequences, `cu_seqlens` with 5 start/end positions are expected
>>> cu_seqlens = q.new_tensor([0, 2048, 4096, 6144, 8192], dtype=torch.long)
>>> o, ht = chunk_delta_rule(
q, k, v, beta,
initial_state=h0,
output_final_state=True,
cu_seqlens=cu_seqlens
)
"""
assert q.dtype == k.dtype == v.dtype
assert q.dtype != torch.float32, "ChunkDeltaRuleFunction does not support float32. Please use bfloat16."
assert len(beta.shape) == 3, "beta must be of shape (batch size, num of head, seq len)."
if head_first:
raise DeprecationWarning(
"head_first is deprecated and will be removed in a future version. "
"Please use head_first=False for now instead.",
)
if not head_first and q.shape[1] < q.shape[2]:
warnings.warn(
f"Input tensor shape suggests potential format mismatch: seq_len ({q.shape[1]}) < num_heads ({q.shape[2]}). "
"This may indicate the inputs were passed in head-first format [B, H, T, ...] "
"when head_first=False was specified. "
"Please verify your input tensor format matches the expected shape [B, T, H, ...].",
)
if cu_seqlens is not None:
if q.shape[0] != 1:
raise ValueError(
f"The batch size is expected to be 1 rather than {q.shape[0]} when using `cu_seqlens`."
f"Please flatten variable-length inputs before processing.",
)
if initial_state is not None and initial_state.shape[0] != len(cu_seqlens) - 1:
raise ValueError(
f"The number of initial states is expected to be equal to the number of input sequences, "
f"i.e., {len(cu_seqlens) - 1} rather than {initial_state.shape[0]}.",
)
scale = k.shape[-1] ** -0.5 if scale is None else scale
o, final_state = ChunkDeltaRuleFunction.apply(
q,
k,
v,
beta,
scale,
initial_state,
output_final_state,
use_qk_l2norm_in_kernel,
cu_seqlens,
)
return o, final_state
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