densenet代码解读

densenet代码解读

概述

densenet是一篇受到了resnet启发的文章，它将resnet跳跃连接的思想发扬光大，在输出层不仅会加上输入层的信息，而且将“连接”做到极致，在每一个block里面，每一层的输出都会连接到后一层的输入，充分利用前面得到的特征图。

densenet网络结构图

单个denseblock的结构图

一个具有3个dense block的densenet网络的结构图如下所示：

densenet网络架构参数

densenet代码细节分析

import re
import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.utils.checkpoint as cp
from collections import OrderedDict
# from .utils import load_state_dict_from_url
from torch import Tensor
from typing import Any, List, Tuple
from torchsummary import summary
# 可选择的densenet模型
__all__ = ['DenseNet', 'densenet121', 'densenet169', 'densenet201', 'densenet161']
# 可下载的densenet预训练权重
model_urls = {
    'densenet121': 'https://download.pytorch.org/models/densenet121-a639ec97.pth',
    'densenet169': 'https://download.pytorch.org/models/densenet169-b2777c0a.pth',
    'densenet201': 'https://download.pytorch.org/models/densenet201-c1103571.pth',
    'densenet161': 'https://download.pytorch.org/models/densenet161-8d451a50.pth',
}

# 定义一个denseblock(dense layer),其中growth_rate的意思是一层产生多少个特征图
class _DenseLayer(nn.Module):
    def __init__(
        self,
        num_input_features: int,
        growth_rate: int,
        bn_size: int,
        drop_rate: float,
        memory_efficient: bool = False
    ) -> None:
        super(_DenseLayer, self).__init__()
        # 首先对输入做一次bn、激活、卷积
        self.norm1: nn.BatchNorm2d
        self.add_module('norm1', nn.BatchNorm2d(num_input_features))
        self.relu1: nn.ReLU
        self.add_module('relu1', nn.ReLU(inplace=True))
        self.conv1: nn.Conv2d
        # 输出特征图的数量为bn_size*growth_rate，卷积、bn、激活
        self.add_module('conv1', nn.Conv2d(num_input_features, bn_size *growth_rate, kernel_size=1, stride=1,bias=False))
        self.norm2: nn.BatchNorm2d
        self.add_module('norm2', nn.BatchNorm2d(bn_size * growth_rate))
        self.relu2: nn.ReLU
        self.add_module('relu2', nn.ReLU(inplace=True))
        self.conv2: nn.Conv2d
        self.add_module('conv2', nn.Conv2d(bn_size * growth_rate, growth_rate,kernel_size=3, stride=1, padding=1,bias=False))
        self.drop_rate = float(drop_rate)
        self.memory_efficient = memory_efficient

    def bn_function(self, inputs: List[Tensor]) -> Tensor:
        concated_features = torch.cat(inputs, 1)
        bottleneck_output = self.conv1(self.relu1(self.norm1(concated_features)))  # noqa: T484
        return bottleneck_output

    # 判断当前tensor是否参与梯度传播
    def any_requires_grad(self, input: List[Tensor]) -> bool:
        for tensor in input:
            if tensor.requires_grad:
                return True
        return False

    # @torch.jit.unused  # noqa: T484
    def call_checkpoint_bottleneck(self, input: List[Tensor]) -> Tensor:
        def closure(*inputs):
            return self.bn_function(inputs)

        return cp.checkpoint(closure, *input)

    # @torch.jit._overload_method  # noqa: F811
    def forward(self, input: List[Tensor]) -> Tensor:
        pass

    # @torch.jit._overload_method  # noqa: F811
    def forward(self, input: Tensor) -> Tensor:
        pass

    # torchscript does not yet support *args, so we overload method
    # allowing it to take either a List[Tensor] or single Tensor
    def forward(self, input: Tensor) -> Tensor:  # noqa: F811
        if isinstance(input, Tensor):
            prev_features = [input]
        else:
            prev_features = input

        if self.memory_efficient and self.any_requires_grad(prev_features):
            if torch.jit.is_scripting():
                raise Exception("Memory Efficient not supported in JIT")

            bottleneck_output = self.call_checkpoint_bottleneck(prev_features)
        else:
            bottleneck_output = self.bn_function(prev_features)

        new_features = self.conv2(self.relu2(self.norm2(bottleneck_output)))
        # 加上dropout
        if self.drop_rate > 0:
            new_features = F.dropout(new_features, p=self.drop_rate,training=self.training)
        return new_features


class _DenseBlock(nn.ModuleDict):
    _version = 2

    def __init__(
        self,
        num_layers: int,
        num_input_features: int,
        bn_size: int,
        growth_rate: int,
        drop_rate: float,
        memory_efficient: bool = False
    ) -> None:
        super(_DenseBlock, self).__init__()
        # 随着layer层数的增加，每增加一层，输入的特征图就增加一倍growth_rate
        for i in range(num_layers):
            layer = _DenseLayer(
                num_input_features + i * growth_rate,
                growth_rate=growth_rate,
                bn_size=bn_size,
                drop_rate=drop_rate,
                memory_efficient=memory_efficient,
            )
            # 添加一层layer
            self.add_module('denselayer%d' % (i + 1), layer)

    def forward(self, init_features: Tensor) -> Tensor:
        # 提取特征
        features = [init_features]
        for name, layer in self.items():
            new_features = layer(features)
            features.append(new_features)
        # 将特征图concat在一起
        return torch.cat(features, 1)


class _Transition(nn.Sequential):
    def __init__(self, num_input_features: int, num_output_features: int) -> None:
        super(_Transition, self).__init__()
        # transition层使用的是1 x 1卷积核，作用是用来改变通道数
        self.add_module('norm', nn.BatchNorm2d(num_input_features))
        self.add_module('relu', nn.ReLU(inplace=True))
        self.add_module('conv', nn.Conv2d(num_input_features, num_output_features,kernel_size=1, stride=1, bias=False))
        self.add_module('pool', nn.AvgPool2d(kernel_size=2, stride=2))


class DenseNet(nn.Module):
    def __init__(
        self,
        growth_rate: int = 32,
        block_config: Tuple[int, int, int, int] = (6, 12, 24, 16),
        num_init_features: int = 64,
        bn_size: int = 4,
        drop_rate: float = 0,
        num_classes: int = 1000,
        memory_efficient: bool = False
    ) -> None:

        super(DenseNet, self).__init__()

        # 第一层卷积
        self.features = nn.Sequential(OrderedDict([
            ('conv0', nn.Conv2d(3, num_init_features, kernel_size=7, stride=2,
                                padding=3, bias=False)),
            ('norm0', nn.BatchNorm2d(num_init_features)),
            ('relu0', nn.ReLU(inplace=True)),
            ('pool0', nn.MaxPool2d(kernel_size=3, stride=2, padding=1)),
        ]))

        # 构建每一个denseblock
        num_features = num_init_features
        for i, num_layers in enumerate(block_config):
            block = _DenseBlock(
                num_layers=num_layers,
                num_input_features=num_features,
                bn_size=bn_size,
                growth_rate=growth_rate,
                drop_rate=drop_rate,
                memory_efficient=memory_efficient
            )
            self.features.add_module('denseblock%d' % (i + 1), block)
            num_features = num_features + num_layers * growth_rate
            if i != len(block_config) - 1:
                trans = _Transition(num_input_features=num_features,
                                    num_output_features=num_features // 2)
                self.features.add_module('transition%d' % (i + 1), trans)
                num_features = num_features // 2

        # 最后一个bn层
        self.features.add_module('norm5', nn.BatchNorm2d(num_features))

        # 分类器
        self.classifier = nn.Linear(num_features, num_classes)

        # 参数初始化
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(m.weight)
            elif isinstance(m, nn.BatchNorm2d):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)
            elif isinstance(m, nn.Linear):
                nn.init.constant_(m.bias, 0)

    def forward(self, x: Tensor) -> Tensor:
        # 提取特征、激活、池化、摊平、分类
        features = self.features(x)
        out = F.relu(features, inplace=True)
        out = F.adaptive_avg_pool2d(out, (1, 1))
        out = torch.flatten(out, 1)
        out = self.classifier(out)
        return out

# 加载训练权重
def _load_state_dict(model: nn.Module, model_url: str, progress: bool) -> None:
    # '.'s are no longer allowed in module names, but previous _DenseLayer
    # has keys 'norm.1', 'relu.1', 'conv.1', 'norm.2', 'relu.2', 'conv.2'.
    # They are also in the checkpoints in model_urls. This pattern is used
    # to find such keys.
    pattern = re.compile(
        r'^(.*denselayer\d+\.(?:norm|relu|conv))\.((?:[12])\.(?:weight|bias|running_mean|running_var))$')

    state_dict = load_state_dict_from_url(model_url, progress=progress)
    for key in list(state_dict.keys()):
        res = pattern.match(key)
        if res:
            new_key = res.group(1) + res.group(2)
            state_dict[new_key] = state_dict[key]
            del state_dict[key]
    model.load_state_dict(state_dict)


def _densenet(
    arch: str,
    growth_rate: int,
    block_config: Tuple[int, int, int, int],
    num_init_features: int,
    pretrained: bool,
    progress: bool,
    **kwargs: Any
) -> DenseNet:
    model = DenseNet(growth_rate, block_config, num_init_features, **kwargs)
    if pretrained:
        _load_state_dict(model, model_urls[arch], progress)
    return model

# 预训练权重，其中第一个参数'densenet121'代表densenet的模型名称，32代表每一层添加32个特征图，(6, 12, 24, 16)表示4个denselayer重复的次数，64表示初始特征数
def densenet121(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> DenseNet:
    return _densenet('densenet121', 32, (6, 12, 24, 16), 64, pretrained, progress,
                     **kwargs)


def densenet161(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> DenseNet:
    return _densenet('densenet161', 48, (6, 12, 36, 24), 96, pretrained, progress,
                     **kwargs)


def densenet169(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> DenseNet:
    return _densenet('densenet169', 32, (6, 12, 32, 32), 64, pretrained, progress,
                     **kwargs)


def densenet201(pretrained: bool = False, progress: bool = True, **kwargs: Any) -> DenseNet:
    return _densenet('densenet201', 32, (6, 12, 48, 32), 64, pretrained, progress,
                     **kwargs)