CNN模型分析 | 5 利用Pytorch復現VGG-16網絡

利用Pytorch復現VGG-16網絡:

根據吳恩達老師在深度學習課程中的講解，AlexNet網絡的基本流程爲：

代碼如下：

import math
import torch
import torchvision
import torch.nn as nn
import torch.nn.functional as F
import torchvision.models as models
from torch.autograd import Variable

class VGG16(nn.Module):
    def __init__(self, num_classes):
        super(VGG16, self).__init__()
        self.feature = nn.Sequential(
            nn.Conv2d(in_channels = 3, out_channels = 64, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 64, out_channels = 64, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(2, 2),
            nn.Conv2d(in_channels = 64, out_channels = 128, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 128, out_channels = 128, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(2, 2),
            nn.Conv2d(in_channels = 128, out_channels = 256, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 256, out_channels = 256, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 256, out_channels = 256, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(2, 2),
            nn.Conv2d(in_channels = 256, out_channels = 512, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 512, out_channels = 512, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 512, out_channels = 512, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(2, 2),
            nn.Conv2d(in_channels = 512, out_channels = 512, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 512, out_channels = 512, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.Conv2d(in_channels = 512, out_channels = 512, kernel_size = 3, stride=1, padding=1),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(2, 2),
        )
            
        self.classifier = nn.Sequential(
            nn.Linear(512 * 7 * 7, 4096),
            nn.ReLU(inplace=True),
            nn.Dropout(),
            nn.Linear(4096, 4096),
            nn.ReLU(inplace=True),
            nn.Dropout(),
            nn.Linear(4096, num_classes)
        )
        self._initialize_weights()
            
    def forward(self, x):
        x = self.feature(x)
        x = x.view(x.size(0), -1)
        x = self.classifier(x)
        return(x)
    
    def _initialize_weights(self):
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                n = m.kernel_size[0] * m.kernel_size[1] * m.out_channels
                m.weight.data.normal_(0, math.sqrt(2. / n))
                if m.bias is not None:
                    m.bias.data.zero_()
            elif isinstance(m, nn.BatchNorm2d):
                m.weight.data.fill_(1)
                m.bias.data.zero_()
            elif isinstance(m, nn.Linear):
                m.weight.data.normal_(0, 0.01)
                m.bias.data.zero_()
            

net = VGG16(1000)
print(net)

輸出：

VGG16(
  (feature): Sequential(
    (0): Conv2d(3, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (1): ReLU(inplace)
    (2): Conv2d(64, 64, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (3): ReLU(inplace)
    (4): MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
    (5): Conv2d(64, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (6): ReLU(inplace)
    (7): Conv2d(128, 128, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (8): ReLU(inplace)
    (9): MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
    (10): Conv2d(128, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (11): ReLU(inplace)
    (12): Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (13): ReLU(inplace)
    (14): Conv2d(256, 256, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (15): ReLU(inplace)
    (16): MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
    (17): Conv2d(256, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (18): ReLU(inplace)
    (19): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (20): ReLU(inplace)
    (21): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (22): ReLU(inplace)
    (23): MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
    (24): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (25): ReLU(inplace)
    (26): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (27): ReLU(inplace)
    (28): Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
    (29): ReLU(inplace)
    (30): MaxPool2d(kernel_size=2, stride=2, padding=0, dilation=1, ceil_mode=False)
  )
  (classifier): Sequential(
    (0): Linear(in_features=25088, out_features=4096, bias=True)
    (1): ReLU(inplace)
    (2): Dropout(p=0.5)
    (3): Linear(in_features=4096, out_features=4096, bias=True)
    (4): ReLU(inplace)
    (5): Dropout(p=0.5)
    (6): Linear(in_features=4096, out_features=1000, bias=True)
  )
)

留下一個問題：

全連接部分的4096是怎麼來的，爲什麼是這個值呢？