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      VGG全称是Visual Geometry Group，因为是由Oxford的Visual Geometry Group提出的。AlexNet问世之后，很多学者通过改进AlexNet的网络结构来提高自己的准确率，主要有两个方向：小卷积核和多尺度。而VGG的作者们则选择了另外一个方向，即加深网络深度。
      卷积网络的输入是224 * 224的RGB图像，整个网络的组成是非常格式化的，基本上都用的是3 * 3的卷积核以及 2 * 2的max pooling，少部分网络加入了1 * 1的卷积核。因为想要体现出“上下左右中”的概念，3*3的卷积核已经是最小的尺寸了。

import torch
import torch.nn as nn# 定义VGG模型
class VGG(nn.Module):def __init__(self, num_classes=1000):super(VGG, self).__init__()self.features = nn.Sequential(nn.Conv2d(3, 64, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(64, 64, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2, stride=2),nn.Conv2d(64, 128, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(128, 128, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2, stride=2),nn.Conv2d(128, 256, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(256, 256, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(256, 256, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2, stride=2),nn.Conv2d(256, 512, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(512, 512, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(512, 512, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2, stride=2),nn.Conv2d(512, 512, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(512, 512, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(512, 512, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2, stride=2))self.avgpool = nn.AdaptiveAvgPool2d((7, 7))self.classifier = nn.Sequential(nn.Linear(7 * 7 * 512, 4096),nn.ReLU(inplace=True),nn.Dropout(),nn.Linear(4096, 4096),nn.ReLU(inplace=True),nn.Dropout(),nn.Linear(4096, num_classes))def forward(self, x):x = self.features(x)x = self.avgpool(x)x = torch.flatten(x, 1)x = self.classifier(x)return x# 创建VGG模型实例
model = VGG()