轻量级网络论文：Searching for MobileNetV3及其PyTorch实现

Searching for MobileNetV3
https://arxiv.org/abs/1905.02244
[Pytorch] https://github.com/shanglianlm0525/PyTorch-Networks

1 概述

MobileNet V3 = MobileNet v2 + SE结构 + hard-swish activation +网络结构头尾微调。除了激活函数，看不出有什么亮点。

2 网络架构搜索

关于网络架构搜索（NAS）

2-1 模块级的搜索（Block-wise Search）

资源受限的NAS（platform-aware NAS）在资源受限条件下搜索网络的各个模块。

MnasNet: Platform-Aware Neural Architecture Search for Mobile

2-2 层级的搜索（Layer-wise Search）

NetAdapt对各个模块确定之后的网络层进行微调。

Netadapt: Platform-aware neural network adaptation for mobile applications

3 网络设计

3-1 网络结构优化

a. 网络端部加速
砍掉最后的一些层来提速，即先使用global average pooling降低计算代价。

b. 网络头部加速
通道数减半；
使用h-swish代替ReLU 或者 swish；

3-2 激活函数（h-swish）

swish x【1】 能有效改进网络精度，但是比较耗时，

因此提出swish x 的近似版h-swish

h-swish 实现比 swish 快，但是比 relu 还是要慢不少。
【1】 Searching for activation functions

3-3 MobileNetV3 网络结构

4 实验结果

4-1 图像分类（Classification）

4-2 目标检测（Detection）

4-3 语义分割（Semantic Segmentation）

Lite R-ASPP结构：

PyTorch代码:

import torch
import torch.nn as nn
import torchvision

class HardSwish(nn.Module):
    def __init__(self, inplace=True):
        super(HardSwish, self).__init__()
        self.relu6 = nn.ReLU6(inplace)

    def forward(self, x):
        return x*self.relu6(x+3)/6

def ConvBNActivation(in_channels,out_channels,kernel_size,stride,activate):
    return nn.Sequential(
            nn.Conv2d(in_channels=in_channels, out_channels=out_channels, kernel_size=kernel_size, stride=stride, padding=(kernel_size-1)//2, groups=in_channels),
            nn.BatchNorm2d(out_channels),
            nn.ReLU6(inplace=True) if activate == 'relu' else HardSwish()
        )

def Conv1x1BNActivation(in_channels,out_channels,activate):
    return nn.Sequential(
            nn.Conv2d(in_channels=in_channels, out_channels=out_channels, kernel_size=1, stride=1),
            nn.BatchNorm2d(out_channels),
            nn.ReLU6(inplace=True) if activate == 'relu' else HardSwish()
        )

def Conv1x1BN(in_channels,out_channels):
    return nn.Sequential(
            nn.Conv2d(in_channels=in_channels, out_channels=out_channels, kernel_size=1, stride=1),
            nn.BatchNorm2d(out_channels)
        )

class SqueezeAndExcite(nn.Module):
    def __init__(self, in_channels, out_channels,se_kernel_size, divide=4):
        super(SqueezeAndExcite, self).__init__()
        mid_channels = in_channels // divide
        self.pool = nn.AvgPool2d(kernel_size=se_kernel_size,stride=1)
        self.SEblock = nn.Sequential(
            nn.Linear(in_features=in_channels, out_features=mid_channels),
            nn.ReLU6(inplace=True),
            nn.Linear(in_features=mid_channels, out_features=out_channels),
            HardSwish(inplace=True),
        )

    def forward(self, x):
        b, c, h, w = x.size()
        out = self.pool(x)
        out = out.view(b, -1)
        out = self.SEblock(out)
        out = out.view(b, c, 1, 1)
        return out * x

class SEInvertedBottleneck(nn.Module):
    def __init__(self, in_channels, mid_channels, out_channels, kernel_size, stride,activate, use_se, se_kernel_size=1):
        super(SEInvertedBottleneck, self).__init__()
        self.stride = stride
        self.use_se = use_se
        # mid_channels = (in_channels * expansion_factor)

        self.conv = Conv1x1BNActivation(in_channels, mid_channels,activate)
        self.depth_conv = ConvBNActivation(mid_channels, mid_channels, kernel_size,stride,activate)
        if self.use_se:
            self.SEblock = SqueezeAndExcite(mid_channels, mid_channels, se_kernel_size)

        self.point_conv = Conv1x1BNActivation(mid_channels, out_channels,activate)

        if self.stride == 1:
            self.shortcut = Conv1x1BN(in_channels, out_channels)

    def forward(self, x):
        out = self.depth_conv(self.conv(x))
        if self.use_se:
            out = self.SEblock(out)
        out = self.point_conv(out)
        out = (out + self.shortcut(x)) if self.stride == 1 else out
        return out


class MobileNetV3(nn.Module):
    def __init__(self, num_classes=1000,type='large'):
        super(MobileNetV3, self).__init__()
        self.type = type

        self.first_conv = nn.Sequential(
            nn.Conv2d(in_channels=3, out_channels=16, kernel_size=3, stride=2, padding=1),
            nn.BatchNorm2d(16),
            HardSwish(inplace=True),
        )

        if type=='large':
            self.large_bottleneck = nn.Sequential(
                SEInvertedBottleneck(in_channels=16, mid_channels=16, out_channels=16, kernel_size=3, stride=1,activate='relu', use_se=False),
                SEInvertedBottleneck(in_channels=16, mid_channels=64, out_channels=24, kernel_size=3, stride=2, activate='relu', use_se=False),
                SEInvertedBottleneck(in_channels=24, mid_channels=72, out_channels=24, kernel_size=3, stride=1, activate='relu', use_se=False),
                SEInvertedBottleneck(in_channels=24, mid_channels=72, out_channels=40, kernel_size=5, stride=2,activate='relu', use_se=True, se_kernel_size=28),
                SEInvertedBottleneck(in_channels=40, mid_channels=120, out_channels=40, kernel_size=5, stride=1,activate='relu', use_se=True, se_kernel_size=28),
                SEInvertedBottleneck(in_channels=40, mid_channels=120, out_channels=40, kernel_size=5, stride=1,activate='relu', use_se=True, se_kernel_size=28),
                SEInvertedBottleneck(in_channels=40, mid_channels=240, out_channels=80, kernel_size=3, stride=1,activate='hswish', use_se=False),
                SEInvertedBottleneck(in_channels=80, mid_channels=200, out_channels=80, kernel_size=3, stride=1,activate='hswish', use_se=False),
                SEInvertedBottleneck(in_channels=80, mid_channels=184, out_channels=80, kernel_size=3, stride=2,activate='hswish', use_se=False),
                SEInvertedBottleneck(in_channels=80, mid_channels=184, out_channels=80, kernel_size=3, stride=1,activate='hswish', use_se=False),
                SEInvertedBottleneck(in_channels=80, mid_channels=480, out_channels=112, kernel_size=3, stride=1,activate='hswish', use_se=True, se_kernel_size=14),
                SEInvertedBottleneck(in_channels=112, mid_channels=672, out_channels=112, kernel_size=3, stride=1,activate='hswish', use_se=True, se_kernel_size=14),
                SEInvertedBottleneck(in_channels=112, mid_channels=672, out_channels=160, kernel_size=5, stride=2,activate='hswish', use_se=True,se_kernel_size=7),
                SEInvertedBottleneck(in_channels=160, mid_channels=960, out_channels=160, kernel_size=5, stride=1,activate='hswish', use_se=True,se_kernel_size=7),
                SEInvertedBottleneck(in_channels=160, mid_channels=960, out_channels=160, kernel_size=5, stride=1,activate='hswish', use_se=True,se_kernel_size=7),
            )

            self.large_last_stage = nn.Sequential(
                nn.Conv2d(in_channels=160, out_channels=960, kernel_size=1, stride=1),
                nn.BatchNorm2d(960),
                HardSwish(inplace=True),
                nn.AvgPool2d(kernel_size=7, stride=1),
                nn.Conv2d(in_channels=960, out_channels=1280, kernel_size=1, stride=1),
                HardSwish(inplace=True),
            )
        else:
            self.small_bottleneck = nn.Sequential(
                SEInvertedBottleneck(in_channels=16, mid_channels=16, out_channels=16, kernel_size=3, stride=2,activate='relu', use_se=True, se_kernel_size=56),
                SEInvertedBottleneck(in_channels=16, mid_channels=72, out_channels=24, kernel_size=3, stride=2,activate='relu', use_se=False),
                SEInvertedBottleneck(in_channels=24, mid_channels=88, out_channels=24, kernel_size=3, stride=1,activate='relu', use_se=False),
                SEInvertedBottleneck(in_channels=24, mid_channels=96, out_channels=40, kernel_size=5, stride=2,activate='hswish', use_se=True, se_kernel_size=14),
                SEInvertedBottleneck(in_channels=40, mid_channels=240, out_channels=40, kernel_size=5, stride=1,activate='hswish', use_se=True, se_kernel_size=14),
                SEInvertedBottleneck(in_channels=40, mid_channels=240, out_channels=40, kernel_size=5, stride=1,activate='hswish', use_se=True, se_kernel_size=14),
                SEInvertedBottleneck(in_channels=40, mid_channels=120, out_channels=48, kernel_size=5, stride=1,activate='hswish', use_se=True, se_kernel_size=14),
                SEInvertedBottleneck(in_channels=48, mid_channels=144, out_channels=48, kernel_size=5, stride=1,activate='hswish', use_se=True, se_kernel_size=14),
                SEInvertedBottleneck(in_channels=48, mid_channels=288, out_channels=96, kernel_size=5, stride=2,activate='hswish', use_se=True, se_kernel_size=7),
                SEInvertedBottleneck(in_channels=96, mid_channels=576, out_channels=96, kernel_size=5, stride=1,activate='hswish', use_se=True, se_kernel_size=7),
                SEInvertedBottleneck(in_channels=96, mid_channels=576, out_channels=96, kernel_size=5, stride=1,activate='hswish', use_se=True, se_kernel_size=7),
            )
            self.small_last_stage = nn.Sequential(
                nn.Conv2d(in_channels=96, out_channels=576, kernel_size=1, stride=1),
                nn.BatchNorm2d(576),
                HardSwish(inplace=True),
                nn.AvgPool2d(kernel_size=7, stride=1),
                nn.Conv2d(in_channels=576, out_channels=1280, kernel_size=1, stride=1),
                HardSwish(inplace=True),
            )

        self.classifier = nn.Conv2d(in_channels=1280, out_channels=num_classes, kernel_size=1, stride=1)

    def init_params(self):
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(m.weight)
                nn.init.constant_(m.bias, 0)
            elif isinstance(m, nn.BatchNorm2d) or isinstance(m, nn.Linear):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)

    def forward(self, x):
        x = self.first_conv(x)
        if self.type == 'large':
            x = self.large_bottleneck(x)
            x = self.large_last_stage(x)
        else:
            x = self.small_bottleneck(x)
            x = self.small_last_stage(x)
        out = self.classifier(x)
        out = out.view(out.size(0), -1)
        return out

if __name__ == '__main__':
    model = MobileNetV3(type='small')
    print(model)

    input = torch.randn(1, 3, 224, 224)
    out = model(input)
    print(out.shape)