mmdet/models/backbones/trident_resnet.py

import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.utils.checkpoint as cp
from mmcv.cnn import build_conv_layer, build_norm_layer, kaiming_init
from torch.nn.modules.utils import _pair

from mmdet.models.backbones.resnet import Bottleneck, ResNet
from mmdet.models.builder import BACKBONES


class TridentConv(nn.Module):
    """Trident Convolution Module.

    Args:
        in_channels (int): Number of channels in input.
        out_channels (int): Number of channels in output.
        kernel_size (int): Size of convolution kernel.
        stride (int, optional): Convolution stride. Default: 1.
        trident_dilations (tuple[int, int, int], optional): Dilations of
            different trident branch. Default: (1, 2, 3).
        test_branch_idx (int, optional): In inference, all 3 branches will
            be used if `test_branch_idx==-1`, otherwise only branch with
            index `test_branch_idx` will be used. Default: 1.
        bias (bool, optional): Whether to use bias in convolution or not.
            Default: False.
    """

    def __init__(self,
                 in_channels,
                 out_channels,
                 kernel_size,
                 stride=1,
                 trident_dilations=(1, 2, 3),
                 test_branch_idx=1,
                 bias=False):
        super(TridentConv, self).__init__()
        self.num_branch = len(trident_dilations)
        self.with_bias = bias
        self.test_branch_idx = test_branch_idx
        self.stride = _pair(stride)
        self.kernel_size = _pair(kernel_size)
        self.paddings = _pair(trident_dilations)
        self.dilations = trident_dilations
        self.in_channels = in_channels
        self.out_channels = out_channels
        self.bias = bias

        self.weight = nn.Parameter(
            torch.Tensor(out_channels, in_channels, *self.kernel_size))
        if bias:
            self.bias = nn.Parameter(torch.Tensor(out_channels))
        else:
            self.bias = None
        self.init_weights()

    def init_weights(self):
        kaiming_init(self, distribution='uniform', mode='fan_in')

    def extra_repr(self):
        tmpstr = f'in_channels={self.in_channels}'
        tmpstr += f', out_channels={self.out_channels}'
        tmpstr += f', kernel_size={self.kernel_size}'
        tmpstr += f', num_branch={self.num_branch}'
        tmpstr += f', test_branch_idx={self.test_branch_idx}'
        tmpstr += f', stride={self.stride}'
        tmpstr += f', paddings={self.paddings}'
        tmpstr += f', dilations={self.dilations}'
        tmpstr += f', bias={self.bias}'
        return tmpstr

    def forward(self, inputs):
        if self.training or self.test_branch_idx == -1:
            outputs = [
                F.conv2d(input, self.weight, self.bias, self.stride, padding,
                         dilation) for input, dilation, padding in zip(
                             inputs, self.dilations, self.paddings)
            ]
        else:
            assert len(inputs) == 1
            outputs = [
                F.conv2d(inputs[0], self.weight, self.bias, self.stride,
                         self.paddings[self.test_branch_idx],
                         self.dilations[self.test_branch_idx])
            ]

        return outputs


# Since TridentNet is defined over ResNet50 and ResNet101, here we
# only support TridentBottleneckBlock.
class TridentBottleneck(Bottleneck):
    """BottleBlock for TridentResNet.

    Args:
        trident_dilations (tuple[int, int, int]): Dilations of different
            trident branch.
        test_branch_idx (int): In inference, all 3 branches will be used
            if `test_branch_idx==-1`, otherwise only branch with index
            `test_branch_idx` will be used.
        concat_output (bool): Whether to concat the output list to a Tensor.
            `True` only in the last Block.
    """

    def __init__(self, trident_dilations, test_branch_idx, concat_output,
                 **kwargs):

        super(TridentBottleneck, self).__init__(**kwargs)
        self.trident_dilations = trident_dilations
        self.num_branch = len(trident_dilations)
        self.concat_output = concat_output
        self.test_branch_idx = test_branch_idx
        self.conv2 = TridentConv(
            self.planes,
            self.planes,
            kernel_size=3,
            stride=self.conv2_stride,
            bias=False,
            trident_dilations=self.trident_dilations,
            test_branch_idx=test_branch_idx)

    def forward(self, x):

        def _inner_forward(x):
            num_branch = (
                self.num_branch
                if self.training or self.test_branch_idx == -1 else 1)
            identity = x
            if not isinstance(x, list):
                x = (x, ) * num_branch
                identity = x
                if self.downsample is not None:
                    identity = [self.downsample(b) for b in x]

            out = [self.conv1(b) for b in x]
            out = [self.norm1(b) for b in out]
            out = [self.relu(b) for b in out]

            if self.with_plugins:
                for k in range(len(out)):
                    out[k] = self.forward_plugin(out[k],
                                                 self.after_conv1_plugin_names)

            out = self.conv2(out)
            out = [self.norm2(b) for b in out]
            out = [self.relu(b) for b in out]
            if self.with_plugins:
                for k in range(len(out)):
                    out[k] = self.forward_plugin(out[k],
                                                 self.after_conv2_plugin_names)

            out = [self.conv3(b) for b in out]
            out = [self.norm3(b) for b in out]

            if self.with_plugins:
                for k in range(len(out)):
                    out[k] = self.forward_plugin(out[k],
                                                 self.after_conv3_plugin_names)

            out = [
                out_b + identity_b for out_b, identity_b in zip(out, identity)
            ]
            return out

        if self.with_cp and x.requires_grad:
            out = cp.checkpoint(_inner_forward, x)
        else:
            out = _inner_forward(x)

        out = [self.relu(b) for b in out]
        if self.concat_output:
            out = torch.cat(out, dim=0)
        return out


def make_trident_res_layer(block,
                           inplanes,
                           planes,
                           num_blocks,
                           stride=1,
                           trident_dilations=(1, 2, 3),
                           style='pytorch',
                           with_cp=False,
                           conv_cfg=None,
                           norm_cfg=dict(type='BN'),
                           dcn=None,
                           plugins=None,
                           test_branch_idx=-1):
    """Build Trident Res Layers."""

    downsample = None
    if stride != 1 or inplanes != planes * block.expansion:
        downsample = []
        conv_stride = stride
        downsample.extend([
            build_conv_layer(
                conv_cfg,
                inplanes,
                planes * block.expansion,
                kernel_size=1,
                stride=conv_stride,
                bias=False),
            build_norm_layer(norm_cfg, planes * block.expansion)[1]
        ])
        downsample = nn.Sequential(*downsample)

    layers = []
    for i in range(num_blocks):
        layers.append(
            block(
                inplanes=inplanes,
                planes=planes,
                stride=stride if i == 0 else 1,
                trident_dilations=trident_dilations,
                downsample=downsample if i == 0 else None,
                style=style,
                with_cp=with_cp,
                conv_cfg=conv_cfg,
                norm_cfg=norm_cfg,
                dcn=dcn,
                plugins=plugins,
                test_branch_idx=test_branch_idx,
                concat_output=True if i == num_blocks - 1 else False))
        inplanes = planes * block.expansion
    return nn.Sequential(*layers)


@BACKBONES.register_module()
class TridentResNet(ResNet):
    """The stem layer, stage 1 and stage 2 in Trident ResNet are identical to
    ResNet, while in stage 3, Trident BottleBlock is utilized to replace the
    normal BottleBlock to yield trident output. Different branch shares the
    convolution weight but uses different dilations to achieve multi-scale
    output.

                               / stage3(b0) \
    x - stem - stage1 - stage2 - stage3(b1) - output
                               \ stage3(b2) /

    Args:
        depth (int): Depth of resnet, from {50, 101, 152}.
        num_branch (int): Number of branches in TridentNet.
        test_branch_idx (int): In inference, all 3 branches will be used
            if `test_branch_idx==-1`, otherwise only branch with index
            `test_branch_idx` will be used.
        trident_dilations (tuple[int]): Dilations of different trident branch.
            len(trident_dilations) should be equal to num_branch.
    """  # noqa

    def __init__(self, depth, num_branch, test_branch_idx, trident_dilations,
                 **kwargs):

        assert num_branch == len(trident_dilations)
        assert depth in (50, 101, 152)
        super(TridentResNet, self).__init__(depth, **kwargs)
        assert self.num_stages == 3
        self.test_branch_idx = test_branch_idx
        self.num_branch = num_branch

        last_stage_idx = self.num_stages - 1
        stride = self.strides[last_stage_idx]
        dilation = trident_dilations
        dcn = self.dcn if self.stage_with_dcn[last_stage_idx] else None
        if self.plugins is not None:
            stage_plugins = self.make_stage_plugins(self.plugins,
                                                    last_stage_idx)
        else:
            stage_plugins = None
        planes = self.base_channels * 2**last_stage_idx
        res_layer = make_trident_res_layer(
            TridentBottleneck,
            inplanes=(self.block.expansion * self.base_channels *
                      2**(last_stage_idx - 1)),
            planes=planes,
            num_blocks=self.stage_blocks[last_stage_idx],
            stride=stride,
            trident_dilations=dilation,
            style=self.style,
            with_cp=self.with_cp,
            conv_cfg=self.conv_cfg,
            norm_cfg=self.norm_cfg,
            dcn=dcn,
            plugins=stage_plugins,
            test_branch_idx=self.test_branch_idx)

        layer_name = f'layer{last_stage_idx + 1}'

        self.__setattr__(layer_name, res_layer)
        self.res_layers.pop(last_stage_idx)
        self.res_layers.insert(last_stage_idx, layer_name)

        self._freeze_stages()