请详细解析一下python代码: import torch.nn as nn import torch.nn.functional as F class Net(nn.Module): def __init__(self): super(Net, self).__init__() self.conv1 = nn.Conv2d(3, 128, 5, padding=2) self.conv2 = nn.Conv2d(128, 128, 5, padding=2) self.conv3 = nn.Conv2d(128, 256, 3, padding=1) self.conv4 = nn.Conv2d(256, 256, 3, padding=1) self.pool = nn.MaxPool2d(2, 2) self.bn_conv1 = nn.BatchNorm2d(128) self.bn_conv2 = nn.BatchNorm2d(128) self.bn_conv3 = nn.BatchNorm2d(256) self.bn_conv4 = nn.BatchNorm2d(256) self.bn_dense1 = nn.BatchNorm1d(1024) self.bn_dense2 = nn.BatchNorm1d(512) self.dropout_conv = nn.Dropout2d(p=0.25) self.dropout = nn.Dropout(p=0.5) self.fc1 = nn.Linear(256 * 8 * 8, 1024) self.fc2 = nn.Linear(1024, 512) self.fc3 = nn.Linear(512, 10) def conv_layers(self, x): out = F.relu(self.bn_conv1(self.conv1(x))) out = F.relu(self.bn_conv2(self.conv2(out))) out = self.pool(out) out = self.dropout_conv(out) out = F.relu(self.bn_conv3(self.conv3(out))) out = F.relu(self.bn_conv4(self.conv4(out))) out = self.pool(out) out = self.dropout_conv(out) return out def dense_layers(self, x): out = F.relu(self.bn_dense1(self.fc1(x))) out = self.dropout(out) out = F.relu(self.bn_dense2(self.fc2(out))) out = self.dropout(out) out = self.fc3(out) return out def forward(self, x): out = self.conv_layers(x) out = out.view(-1, 256 * 8 * 8) out = self.dense_layers(out) return out net = Net() device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu") print('Device:', device) net.to(device) num_params = sum(p.numel() for p in net.parameters() if p.requires_grad) print("Number of trainable parameters:", num_params)

这段代码实现了一个卷积神经网络，并使用了PyTorch框架中的nn.Module来定义网络结构。该网络由四个卷积层和三个全连接层构成，其中每个卷积层后面都跟着一个Batch Normalization层。同时，为了减少过拟合，该网络还使用了dropout技术在卷积层和全连接层之间添加了一个dropout层。

在该网络中，conv_layers函数用于定义卷积层和Batch Normalization层，并使用了ReLU激活函数和最大池化层。dense_layers函数用于定义全连接层和dropout层，并同样使用ReLU激活函数。最后，forward函数将卷积层和全连接层连接起来，完成整个网络的前向传播。

在代码的最后，网络被实例化为net，并将其移动到GPU上（如果GPU可用）。最后，使用num_params计算网络中可训练参数的数量，并将其打印出来。