在使用Pytorch搭建自己的神经网络框架时，经常需要使用Pytorch中内置的torchvision.models中的模型作为特征提取的Backbone，然后再在这个基础上进行更加复杂的网络搭建。

在这里以使用Pytorch中内置的Resnet18为例，如何作为Backbone层进行使用，看以下示例代码

# -*- coding: utf-8 -*-
import torch.nn as nn
import torchvision

class Resnet18Backbone(nn.Module):
    def __init__(self):
        super(Resnet18Backbone, self).__init__()

        self.model = torchvision.models.resnet18(pretrained=True)
        self.model.fc = nn.Sequential()

    def forward(self, x):
        x = self.model.conv1(x)
        x = self.model.bn1(x)
        x = self.model.relu(x)
        x = self.model.maxpool(x)
        x = self.model.layer1(x)
        x = self.model.layer2(x)
        x = self.model.layer3(x)
        x = self.model.layer4(x)
        x = self.model.avgpool(x)

        return x

使用上述代码，如果输入Tensor的维度为[1,3,244,244]，fowward输出的Tensor的维度为[1,512,1,1]，如果我们需要输出的Tensor维度为[1,512]，需要squeeze相应的维度，修改后的代码如下

# -*- coding: utf-8 -*-
import torch.nn as nn
import torchvision

class Resnet18Backbone(nn.Module):
    def __init__(self):
        super(Resnet18Backbone, self).__init__()

        self.model = torchvision.models.resnet18(pretrained=True)
        self.model.fc = nn.Sequential()

    def forward(self, x):
        x = self.model.conv1(x)
        x = self.model.bn1(x)
        x = self.model.relu(x)
        x = self.model.maxpool(x)
        x = self.model.layer1(x)
        x = self.model.layer2(x)
        x = self.model.layer3(x)
        x = self.model.layer4(x)
        x = self.model.avgpool(x)
        x = x.squeeze(2).squeeze(2)

        return x

好了，上述代码的Resnet18Backbone可以作为网络中的一层进行使用，这里都是以ResNet的Adaptive Average Pooling层作为backbone的输出层，如果我们仅仅需要前面的卷积层作为输出层，可以参考以下代码。

比如，如果我们要使用ResNet18的Adaptive Average Pooling作为backbone的输出层，我们可以这样写，

# backbone
        if backbone_name == 'resnet_18':
            resnet_net = torchvision.models.resnet18(pretrained=True)
            modules = list(resnet_net.children())[:-1]
            backbone = nn.Sequential(*modules)
            backbone.out_channels = 512
        elif backbone_name == 'resnet_34':
            resnet_net = torchvision.models.resnet34(pretrained=True)
            modules = list(resnet_net.children())[:-1]
            backbone = nn.Sequential(*modules)
            backbone.out_channels = 512
        elif backbone_name == 'resnet_50':
            resnet_net = torchvision.models.resnet50(pretrained=True)
            modules = list(resnet_net.children())[:-1]
            backbone = nn.Sequential(*modules)
            backbone.out_channels = 2048
        elif backbone_name == 'resnet_101':
            resnet_net = torchvision.models.resnet101(pretrained=True)
            modules = list(resnet_net.children())[:-1]
            backbone = nn.Sequential(*modules)
            backbone.out_channels = 2048
        elif backbone_name == 'resnet_152':
            resnet_net = torchvision.models.resnet152(pretrained=True)
            modules = list(resnet_net.children())[:-1]
            backbone = nn.Sequential(*modules)
            backbone.out_channels = 2048
        elif backbone_name == 'resnet_50_modified_stride_1':
            resnet_net = resnet50(pretrained=True)
            modules = list(resnet_net.children())[:-1]
            backbone = nn.Sequential(*modules)
            backbone.out_channels = 2048
        elif backbone_name == 'resnext101_32x8d':
            resnet_net = torchvision.models.resnext101_32x8d(pretrained=True)
            modules = list(resnet_net.children())[:-1]
            backbone = nn.Sequential(*modules)
            backbone.out_channels = 2048

如果我们仅仅只是需要前面的卷积层作为backbone，我们可以这样写

# backbone
        if backbone_name == 'resnet_18':
            resnet_net = torchvision.models.resnet18(pretrained=True)
            modules = list(resnet_net.children())[:-2]
            backbone = nn.Sequential(*modules)

        elif backbone_name == 'resnet_34':
            resnet_net = torchvision.models.resnet34(pretrained=True)
            modules = list(resnet_net.children())[:-2]
            backbone = nn.Sequential(*modules)

        elif backbone_name == 'resnet_50':
            resnet_net = torchvision.models.resnet50(pretrained=True)
            modules = list(resnet_net.children())[:-2]
            backbone = nn.Sequential(*modules)

        elif backbone_name == 'resnet_101':
            resnet_net = torchvision.models.resnet101(pretrained=True)
            modules = list(resnet_net.children())[:-2]
            backbone = nn.Sequential(*modules)

        elif backbone_name == 'resnet_152':
            resnet_net = torchvision.models.resnet152(pretrained=True)
            modules = list(resnet_net.children())[:-2]
            backbone = nn.Sequential(*modules)

        elif backbone_name == 'resnet_50_modified_stride_1':
            resnet_net = resnet50(pretrained=True)
            modules = list(resnet_net.children())[:-2]
            backbone = nn.Sequential(*modules)

        elif backbone_name == 'resnext101_32x8d':
            resnet_net = torchvision.models.resnext101_32x8d(pretrained=True)
            modules = list(resnet_net.children())[:-2]
            backbone = nn.Sequential(*modules)

参考链接