modules/resnet.py

import torch
from torch import nn
import torch.nn.functional as F
#import fvcore.nn.weight_init as weight_init

"""
Functions for building the BottleneckBlock from Detectron2.
# https://github.com/facebookresearch/detectron2/blob/main/detectron2/modeling/backbone/resnet.py
"""

def get_norm(norm, out_channels, num_norm_groups=32):
    """
    Args:
        norm (str or callable): either one of BN, SyncBN, FrozenBN, GN;
            or a callable that takes a channel number and returns
            the normalization layer as a nn.Module.
    Returns:
        nn.Module or None: the normalization layer
    """
    if norm is None:
        return None
    if isinstance(norm, str):
        if len(norm) == 0:
            return None
        norm = {
            "GN": lambda channels: nn.GroupNorm(num_norm_groups, channels),
        }[norm]
    return norm(out_channels)

class Conv2d(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):
        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
    
class CNNBlockBase(nn.Module):
    """
    A CNN block is assumed to have input channels, output channels and a stride.
    The input and output of `forward()` method must be NCHW tensors.
    The method can perform arbitrary computation but must match the given
    channels and stride specification.
    Attribute:
        in_channels (int):
        out_channels (int):
        stride (int):
    """

    def __init__(self, in_channels, out_channels, stride):
        """
        The `__init__` method of any subclass should also contain these arguments.
        Args:
            in_channels (int):
            out_channels (int):
            stride (int):
        """
        super().__init__()
        self.in_channels = in_channels
        self.out_channels = out_channels
        self.stride = stride
    
class BottleneckBlock(CNNBlockBase):
    """
    The standard bottleneck residual block used by ResNet-50, 101 and 152
    defined in :paper:`ResNet`.  It contains 3 conv layers with kernels
    1x1, 3x3, 1x1, and a projection shortcut if needed.
    """

    def __init__(
        self,
        in_channels,
        out_channels,
        *,
        bottleneck_channels,
        stride=1,
        num_groups=1,
        norm="GN",
        stride_in_1x1=False,
        dilation=1,
        num_norm_groups=32
    ):
        """
        Args:
            bottleneck_channels (int): number of output channels for the 3x3
                "bottleneck" conv layers.
            num_groups (int): number of groups for the 3x3 conv layer.
            norm (str or callable): normalization for all conv layers.
                See :func:`layers.get_norm` for supported format.
            stride_in_1x1 (bool): when stride>1, whether to put stride in the
                first 1x1 convolution or the bottleneck 3x3 convolution.
            dilation (int): the dilation rate of the 3x3 conv layer.
        """
        super().__init__(in_channels, out_channels, stride)

        if in_channels != out_channels:
            self.shortcut = Conv2d(
                in_channels,
                out_channels,
                kernel_size=1,
                stride=stride,
                bias=False,
                norm=get_norm(norm, out_channels, num_norm_groups),
            )
        else:
            self.shortcut = None

        # The original MSRA ResNet models have stride in the first 1x1 conv
        # The subsequent fb.torch.resnet and Caffe2 ResNe[X]t implementations have
        # stride in the 3x3 conv
        stride_1x1, stride_3x3 = (stride, 1) if stride_in_1x1 else (1, stride)

        self.conv1 = Conv2d(
            in_channels,
            bottleneck_channels,
            kernel_size=1,
            stride=stride_1x1,
            bias=False,
            norm=get_norm(norm, bottleneck_channels, num_norm_groups),
        )

        self.conv2 = Conv2d(
            bottleneck_channels,
            bottleneck_channels,
            kernel_size=3,
            stride=stride_3x3,
            padding=1 * dilation,
            bias=False,
            groups=num_groups,
            dilation=dilation,
            norm=get_norm(norm, bottleneck_channels, num_norm_groups),
        )

        self.conv3 = Conv2d(
            bottleneck_channels,
            out_channels,
            kernel_size=1,
            bias=False,
            norm=get_norm(norm, out_channels, num_norm_groups),
        )

        #for layer in [self.conv1, self.conv2, self.conv3, self.shortcut]:
        #    if layer is not None:  # shortcut can be None
        #        weight_init.c2_msra_fill(layer)

        # Zero-initialize the last normalization in each residual branch,
        # so that at the beginning, the residual branch starts with zeros,
        # and each residual block behaves like an identity.
        # See Sec 5.1 in "Accurate, Large Minibatch SGD: Training ImageNet in 1 Hour":
        # "For BN layers, the learnable scaling coefficient �� is initialized
        # to be 1, except for each residual block's last BN
        # where �� is initialized to be 0."

        # nn.init.constant_(self.conv3.norm.weight, 0)
        # TODO this somehow hurts performance when training GN models from scratch.
        # Add it as an option when we need to use this code to train a backbone.

    def forward(self, x):
        out = self.conv1(x)
        out = F.relu_(out)

        out = self.conv2(out)
        out = F.relu_(out)

        out = self.conv3(out)

        if self.shortcut is not None:
            shortcut = self.shortcut(x)
        else:
            shortcut = x

        out += shortcut
        out = F.relu_(out)
        return out
    
class ResNet(nn.Module):
    """
    Implement :paper:`ResNet`.
    """

    def __init__(self, stem, stages, num_classes=None, out_features=None, freeze_at=0):
        """
        Args:
            stem (nn.Module): a stem module
            stages (list[list[CNNBlockBase]]): several (typically 4) stages,
                each contains multiple :class:`CNNBlockBase`.
            num_classes (None or int): if None, will not perform classification.
                Otherwise, will create a linear layer.
            out_features (list[str]): name of the layers whose outputs should
                be returned in forward. Can be anything in "stem", "linear", or "res2" ...
                If None, will return the output of the last layer.
            freeze_at (int): The number of stages at the beginning to freeze.
                see :meth:`freeze` for detailed explanation.
        """
        super().__init__()
        self.stem = stem
        self.num_classes = num_classes

        current_stride = self.stem.stride
        self._out_feature_strides = {"stem": current_stride}
        self._out_feature_channels = {"stem": self.stem.out_channels}

        self.stage_names, self.stages = [], []

        if out_features is not None:
            # Avoid keeping unused layers in this module. They consume extra memory
            # and may cause allreduce to fail
            num_stages = max(
                [{"res2": 1, "res3": 2, "res4": 3, "res5": 4}.get(f, 0) for f in out_features]
            )
            stages = stages[:num_stages]
        for i, blocks in enumerate(stages):
            assert len(blocks) > 0, len(blocks)
            for block in blocks:
                assert isinstance(block, CNNBlockBase), block

            name = "res" + str(i + 2)
            stage = nn.Sequential(*blocks)

            self.add_module(name, stage)
            self.stage_names.append(name)
            self.stages.append(stage)

            self._out_feature_strides[name] = current_stride = int(
                current_stride * np.prod([k.stride for k in blocks])
            )
            self._out_feature_channels[name] = curr_channels = blocks[-1].out_channels
        self.stage_names = tuple(self.stage_names)  # Make it static for scripting

        if num_classes is not None:
            self.avgpool = nn.AdaptiveAvgPool2d((1, 1))
            self.linear = nn.Linear(curr_channels, num_classes)

            # Sec 5.1 in "Accurate, Large Minibatch SGD: Training ImageNet in 1 Hour":
            # "The 1000-way fully-connected layer is initialized by
            # drawing weights from a zero-mean Gaussian with standard deviation of 0.01."
            nn.init.normal_(self.linear.weight, std=0.01)
            name = "linear"

        if out_features is None:
            out_features = [name]
        self._out_features = out_features
        assert len(self._out_features)
        children = [x[0] for x in self.named_children()]
        for out_feature in self._out_features:
            assert out_feature in children, "Available children: {}".format(", ".join(children))
        self.freeze(freeze_at)

    def forward(self, x):
        """
        Args:
            x: Tensor of shape (N,C,H,W). H, W must be a multiple of ``self.size_divisibility``.
        Returns:
            dict[str->Tensor]: names and the corresponding features
        """
        assert x.dim() == 4, f"ResNet takes an input of shape (N, C, H, W). Got {x.shape} instead!"
        outputs = {}
        x = self.stem(x)
        if "stem" in self._out_features:
            outputs["stem"] = x
        for name, stage in zip(self.stage_names, self.stages):
            x = stage(x)
            if name in self._out_features:
                outputs[name] = x
        if self.num_classes is not None:
            x = self.avgpool(x)
            x = torch.flatten(x, 1)
            x = self.linear(x)
            if "linear" in self._out_features:
                outputs["linear"] = x
        return outputs

    def freeze(self, freeze_at=0):
        """
        Freeze the first several stages of the ResNet. Commonly used in
        fine-tuning.
        Layers that produce the same feature map spatial size are defined as one
        "stage" by :paper:`FPN`.
        Args:
            freeze_at (int): number of stages to freeze.
                `1` means freezing the stem. `2` means freezing the stem and
                one residual stage, etc.
        Returns:
            nn.Module: this ResNet itself
        """
        if freeze_at >= 1:
            self.stem.freeze()
        for idx, stage in enumerate(self.stages, start=2):
            if freeze_at >= idx:
                for block in stage.children():
                    block.freeze()
        return self

    @staticmethod
    def make_stage(block_class, num_blocks, *, in_channels, out_channels, **kwargs):
        """
        Create a list of blocks of the same type that forms one ResNet stage.
        Args:
            block_class (type): a subclass of CNNBlockBase that's used to create all blocks in this
                stage. A module of this type must not change spatial resolution of inputs unless its
                stride != 1.
            num_blocks (int): number of blocks in this stage
            in_channels (int): input channels of the entire stage.
            out_channels (int): output channels of **every block** in the stage.
            kwargs: other arguments passed to the constructor of
                `block_class`. If the argument name is "xx_per_block", the
                argument is a list of values to be passed to each block in the
                stage. Otherwise, the same argument is passed to every block
                in the stage.
        Returns:
            list[CNNBlockBase]: a list of block module.
        Examples:
        ::
            stage = ResNet.make_stage(
                BottleneckBlock, 3, in_channels=16, out_channels=64,
                bottleneck_channels=16, num_groups=1,
                stride_per_block=[2, 1, 1],
                dilations_per_block=[1, 1, 2]
            )
        Usually, layers that produce the same feature map spatial size are defined as one
        "stage" (in :paper:`FPN`). Under such definition, ``stride_per_block[1:]`` should
        all be 1.
        """
        blocks = []
        for i in range(num_blocks):
            curr_kwargs = {}
            for k, v in kwargs.items():
                if k.endswith("_per_block"):
                    assert len(v) == num_blocks, (
                        f"Argument '{k}' of make_stage should have the "
                        f"same length as num_blocks={num_blocks}."
                    )
                    newk = k[: -len("_per_block")]
                    assert newk not in kwargs, f"Cannot call make_stage with both {k} and {newk}!"
                    curr_kwargs[newk] = v[i]
                else:
                    curr_kwargs[k] = v

            blocks.append(
                block_class(in_channels=in_channels, out_channels=out_channels, **curr_kwargs)
            )
            in_channels = out_channels
        return blocks

    @staticmethod
    def make_default_stages(depth, block_class=None, **kwargs):
        """
        Created list of ResNet stages from pre-defined depth (one of 18, 34, 50, 101, 152).
        If it doesn't create the ResNet variant you need, please use :meth:`make_stage`
        instead for fine-grained customization.
        Args:
            depth (int): depth of ResNet
            block_class (type): the CNN block class. Has to accept
                `bottleneck_channels` argument for depth > 50.
                By default it is BasicBlock or BottleneckBlock, based on the
                depth.
            kwargs:
                other arguments to pass to `make_stage`. Should not contain
                stride and channels, as they are predefined for each depth.
        Returns:
            list[list[CNNBlockBase]]: modules in all stages; see arguments of
                :class:`ResNet.__init__`.
        """
        num_blocks_per_stage = {
            18: [2, 2, 2, 2],
            34: [3, 4, 6, 3],
            50: [3, 4, 6, 3],
            101: [3, 4, 23, 3],
            152: [3, 8, 36, 3],
        }[depth]
        if block_class is None:
            block_class = BasicBlock if depth < 50 else BottleneckBlock
        if depth < 50:
            in_channels = [64, 64, 128, 256]
            out_channels = [64, 128, 256, 512]
        else:
            in_channels = [64, 256, 512, 1024]
            out_channels = [256, 512, 1024, 2048]
        ret = []
        for (n, s, i, o) in zip(num_blocks_per_stage, [1, 2, 2, 2], in_channels, out_channels):
            if depth >= 50:
                kwargs["bottleneck_channels"] = o // 4
            ret.append(
                ResNet.make_stage(
                    block_class=block_class,
                    num_blocks=n,
                    stride_per_block=[s] + [1] * (n - 1),
                    in_channels=i,
                    out_channels=o,
                    **kwargs,
                )
            )
        return ret