detectron2/layers/wrappers.py

# Copyright (c) Facebook, Inc. and its affiliates.
"""
Wrappers around on some nn functions, mainly to support empty tensors.

Ideally, add support directly in PyTorch to empty tensors in those functions.

These can be removed once https://github.com/pytorch/pytorch/issues/12013
is implemented
"""

import warnings
from typing import List, Optional
import torch
from torch.nn import functional as F

from detectron2.utils.env import TORCH_VERSION


def shapes_to_tensor(x: List[int], device: Optional[torch.device] = None) -> torch.Tensor:
    """
    Turn a list of integer scalars or integer Tensor scalars into a vector,
    in a way that's both traceable and scriptable.

    In tracing, `x` should be a list of scalar Tensor, so the output can trace to the inputs.
    In scripting or eager, `x` should be a list of int.
    """
    if torch.jit.is_scripting():
        return torch.as_tensor(x, device=device)
    if torch.jit.is_tracing():
        assert all(
            [isinstance(t, torch.Tensor) for t in x]
        ), "Shape should be tensor during tracing!"
        # as_tensor should not be used in tracing because it records a constant
        ret = torch.stack(x)
        if ret.device != device:  # avoid recording a hard-coded device if not necessary
            ret = ret.to(device=device)
        return ret
    return torch.as_tensor(x, device=device)


def check_if_dynamo_compiling():
    if TORCH_VERSION >= (1, 14):
        from torch._dynamo import is_compiling

        return is_compiling()
    else:
        return False


def cat(tensors: List[torch.Tensor], dim: int = 0):
    """
    Efficient version of torch.cat that avoids a copy if there is only a single element in a list
    """
    assert isinstance(tensors, (list, tuple))
    if len(tensors) == 1:
        return tensors[0]
    return torch.cat(tensors, dim)


def empty_input_loss_func_wrapper(loss_func):
    def wrapped_loss_func(input, target, *, reduction="mean", **kwargs):
        """
        Same as `loss_func`, but returns 0 (instead of nan) for empty inputs.
        """
        if target.numel() == 0 and reduction == "mean":
            return input.sum() * 0.0  # connect the gradient
        return loss_func(input, target, reduction=reduction, **kwargs)

    return wrapped_loss_func


cross_entropy = empty_input_loss_func_wrapper(F.cross_entropy)


class _NewEmptyTensorOp(torch.autograd.Function):
    @staticmethod
    def forward(ctx, x, new_shape):
        ctx.shape = x.shape
        return x.new_empty(new_shape)

    @staticmethod
    def backward(ctx, grad):
        shape = ctx.shape
        return _NewEmptyTensorOp.apply(grad, shape), None


class Conv2d(torch.nn.Conv2d):
    """
    A wrapper around :class:`torch.nn.Conv2d` to support empty inputs and more features.
    """

    def __init__(self, *args, **kwargs):
        """
        Extra keyword arguments supported in addition to those in `torch.nn.Conv2d`:

        Args:
            norm (nn.Module, optional): a normalization layer
            activation (callable(Tensor) -> Tensor): a callable activation function

        It assumes that norm layer is used before activation.
        """
        norm = kwargs.pop("norm", None)
        activation = kwargs.pop("activation", None)
        super().__init__(*args, **kwargs)

        self.norm = norm
        self.activation = activation

    def forward(self, x):
        # torchscript does not support SyncBatchNorm yet
        # https://github.com/pytorch/pytorch/issues/40507
        # and we skip these codes in torchscript since:
        # 1. currently we only support torchscript in evaluation mode
        # 2. features needed by exporting module to torchscript are added in PyTorch 1.6 or
        # later version, `Conv2d` in these PyTorch versions has already supported empty inputs.
        if not torch.jit.is_scripting():
            # Dynamo doesn't support context managers yet
            is_dynamo_compiling = check_if_dynamo_compiling()
            if not is_dynamo_compiling:
                with warnings.catch_warnings(record=True):
                    if x.numel() == 0 and self.training:
                        # https://github.com/pytorch/pytorch/issues/12013
                        assert not isinstance(
                            self.norm, torch.nn.SyncBatchNorm
                        ), "SyncBatchNorm does not support empty inputs!"

        x = F.conv2d(
            x, self.weight, self.bias, self.stride, self.padding, self.dilation, self.groups
        )
        if self.norm is not None:
            x = self.norm(x)
        if self.activation is not None:
            x = self.activation(x)
        return x


ConvTranspose2d = torch.nn.ConvTranspose2d
BatchNorm2d = torch.nn.BatchNorm2d
interpolate = F.interpolate
Linear = torch.nn.Linear


def nonzero_tuple(x):
    """
    A 'as_tuple=True' version of torch.nonzero to support torchscript.
    because of https://github.com/pytorch/pytorch/issues/38718
    """
    if torch.jit.is_scripting():
        if x.dim() == 0:
            return x.unsqueeze(0).nonzero().unbind(1)
        return x.nonzero().unbind(1)
    else:
        return x.nonzero(as_tuple=True)


@torch.jit.script_if_tracing
def move_device_like(src: torch.Tensor, dst: torch.Tensor) -> torch.Tensor:
    """
    Tracing friendly way to cast tensor to another tensor's device. Device will be treated
    as constant during tracing, scripting the casting process as whole can workaround this issue.
    """
    return src.to(dst.device)