model.py

import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.autograd import Variable
from collections import OrderedDict
from torch.nn import init


def conv3x3(in_channels, out_channels, stride=1, 
            padding=1, bias=True, groups=1):    
    """3x3 convolution with padding
    """
    return nn.Conv2d(
        in_channels, 
        out_channels, 
        kernel_size=3, 
        stride=stride,
        padding=padding,
        bias=bias,
        groups=groups)


def conv1x1(in_channels, out_channels, groups=1):
    """1x1 convolution with padding
    - Normal pointwise convolution When groups == 1
    - Grouped pointwise convolution when groups > 1
    """
    return nn.Conv2d(
        in_channels, 
        out_channels, 
        kernel_size=1, 
        groups=groups,
        stride=1)


def channel_shuffle(x, groups):
    batchsize, num_channels, height, width = x.data.size()

    channels_per_group = num_channels // groups
    
    # reshape
    x = x.view(batchsize, groups, 
        channels_per_group, height, width)

    # transpose
    # - contiguous() required if transpose() is used before view().
    #   See https://github.com/pytorch/pytorch/issues/764
    x = torch.transpose(x, 1, 2).contiguous()

    # flatten
    x = x.view(batchsize, -1, height, width)

    return x


class ShuffleUnit(nn.Module):
    def __init__(self, in_channels, out_channels, groups=3,
                 grouped_conv=True, combine='add'):
        
        super(ShuffleUnit, self).__init__()

        self.in_channels = in_channels
        self.out_channels = out_channels
        self.grouped_conv = grouped_conv
        self.combine = combine
        self.groups = groups
        self.bottleneck_channels = self.out_channels // 4

        # define the type of ShuffleUnit
        if self.combine == 'add':
            # ShuffleUnit Figure 2b
            self.depthwise_stride = 1
            self._combine_func = self._add
        elif self.combine == 'concat':
            # ShuffleUnit Figure 2c
            self.depthwise_stride = 2
            self._combine_func = self._concat
            
            # ensure output of concat has the same channels as 
            # original output channels.
            self.out_channels -= self.in_channels
        else:
            raise ValueError("Cannot combine tensors with \"{}\"" \
                             "Only \"add\" and \"concat\" are" \
                             "supported".format(self.combine))

        # Use a 1x1 grouped or non-grouped convolution to reduce input channels
        # to bottleneck channels, as in a ResNet bottleneck module.
        # NOTE: Do not use group convolution for the first conv1x1 in Stage 2.
        self.first_1x1_groups = self.groups if grouped_conv else 1

        self.g_conv_1x1_compress = self._make_grouped_conv1x1(
            self.in_channels,
            self.bottleneck_channels,
            self.first_1x1_groups,
            batch_norm=True,
            relu=True
            )

        # 3x3 depthwise convolution followed by batch normalization
        self.depthwise_conv3x3 = conv3x3(
            self.bottleneck_channels, self.bottleneck_channels,
            stride=self.depthwise_stride, groups=self.bottleneck_channels)
        self.bn_after_depthwise = nn.BatchNorm2d(self.bottleneck_channels)

        # Use 1x1 grouped convolution to expand from 
        # bottleneck_channels to out_channels
        self.g_conv_1x1_expand = self._make_grouped_conv1x1(
            self.bottleneck_channels,
            self.out_channels,
            self.groups,
            batch_norm=True,
            relu=False
            )


    @staticmethod
    def _add(x, out):
        # residual connection
        return x + out


    @staticmethod
    def _concat(x, out):
        # concatenate along channel axis
        return torch.cat((x, out), 1)


    def _make_grouped_conv1x1(self, in_channels, out_channels, groups,
        batch_norm=True, relu=False):

        modules = OrderedDict()

        conv = conv1x1(in_channels, out_channels, groups=groups)
        modules['conv1x1'] = conv

        if batch_norm:
            modules['batch_norm'] = nn.BatchNorm2d(out_channels)
        if relu:
            modules['relu'] = nn.ReLU()
        if len(modules) > 1:
            return nn.Sequential(modules)
        else:
            return conv


    def forward(self, x):
        # save for combining later with output
        residual = x

        if self.combine == 'concat':
            residual = F.avg_pool2d(residual, kernel_size=3, 
                stride=2, padding=1)

        out = self.g_conv_1x1_compress(x)
        out = channel_shuffle(out, self.groups)
        out = self.depthwise_conv3x3(out)
        out = self.bn_after_depthwise(out)
        out = self.g_conv_1x1_expand(out)
        
        out = self._combine_func(residual, out)
        return F.relu(out)


class ShuffleNet(nn.Module):
    """ShuffleNet implementation.
    """

    def __init__(self, groups=3, in_channels=3, num_classes=1000):
        """ShuffleNet constructor.

        Arguments:
            groups (int, optional): number of groups to be used in grouped 
                1x1 convolutions in each ShuffleUnit. Default is 3 for best
                performance according to original paper.
            in_channels (int, optional): number of channels in the input tensor.
                Default is 3 for RGB image inputs.
            num_classes (int, optional): number of classes to predict. Default
                is 1000 for ImageNet.

        """
        super(ShuffleNet, self).__init__()

        self.groups = groups
        self.stage_repeats = [3, 7, 3]
        self.in_channels =  in_channels
        self.num_classes = num_classes

        # index 0 is invalid and should never be called.
        # only used for indexing convenience.
        if groups == 1:
            self.stage_out_channels = [-1, 24, 144, 288, 567]
        elif groups == 2:
            self.stage_out_channels = [-1, 24, 200, 400, 800]
        elif groups == 3:
            self.stage_out_channels = [-1, 24, 240, 480, 960]
        elif groups == 4:
            self.stage_out_channels = [-1, 24, 272, 544, 1088]
        elif groups == 8:
            self.stage_out_channels = [-1, 24, 384, 768, 1536]
        else:
            raise ValueError(
                """{} groups is not supported for
                   1x1 Grouped Convolutions""".format(num_groups))
        
        # Stage 1 always has 24 output channels
        self.conv1 = conv3x3(self.in_channels,
                             self.stage_out_channels[1], # stage 1
                             stride=2)
        self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)

        # Stage 2
        self.stage2 = self._make_stage(2)
        # Stage 3
        self.stage3 = self._make_stage(3)
        # Stage 4
        self.stage4 = self._make_stage(4)

        # Global pooling:
        # Undefined as PyTorch's functional API can be used for on-the-fly
        # shape inference if input size is not ImageNet's 224x224

        # Fully-connected classification layer
        num_inputs = self.stage_out_channels[-1]
        self.fc = nn.Linear(num_inputs, self.num_classes)
        self.init_params()


    def init_params(self):
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                init.kaiming_normal(m.weight, mode='fan_out')
                if m.bias is not None:
                    init.constant(m.bias, 0)
            elif isinstance(m, nn.BatchNorm2d):
                init.constant(m.weight, 1)
                init.constant(m.bias, 0)
            elif isinstance(m, nn.Linear):
                init.normal(m.weight, std=0.001)
                if m.bias is not None:
                    init.constant(m.bias, 0)


    def _make_stage(self, stage):
        modules = OrderedDict()
        stage_name = "ShuffleUnit_Stage{}".format(stage)
        
        # First ShuffleUnit in the stage
        # 1. non-grouped 1x1 convolution (i.e. pointwise convolution)
        #   is used in Stage 2. Group convolutions used everywhere else.
        grouped_conv = stage > 2
        
        # 2. concatenation unit is always used.
        first_module = ShuffleUnit(
            self.stage_out_channels[stage-1],
            self.stage_out_channels[stage],
            groups=self.groups,
            grouped_conv=grouped_conv,
            combine='concat'
            )
        modules[stage_name+"_0"] = first_module

        # add more ShuffleUnits depending on pre-defined number of repeats
        for i in range(self.stage_repeats[stage-2]):
            name = stage_name + "_{}".format(i+1)
            module = ShuffleUnit(
                self.stage_out_channels[stage],
                self.stage_out_channels[stage],
                groups=self.groups,
                grouped_conv=True,
                combine='add'
                )
            modules[name] = module

        return nn.Sequential(modules)


    def forward(self, x):
        x = self.conv1(x)
        x = self.maxpool(x)

        x = self.stage2(x)
        x = self.stage3(x)
        x = self.stage4(x)

        # global average pooling layer
        x = F.avg_pool2d(x, x.data.size()[-2:])
        
        # flatten for input to fully-connected layer
        x = x.view(x.size(0), -1)
        x = self.fc(x)

        return F.log_softmax(x, dim=1)


if __name__ == "__main__":
    """Testing
    """
    model = ShuffleNet()