pytorch Rnn输出大小和教程代码不一样解惑

浅浅的花香味﹌ 2022-03-15 15:48 184阅读 0赞

在学习pytorch时，按照教程搭建了LSTM网络，去识别MINST数据集。

代码如下：

import torch 
    import torch.nn as nn
    import torchvision
    import torchvision.transforms as transforms
    
    
    # Device configuration
    device = torch.device('cuda' if torch.cuda.is_available() else 'cpu')
    
    # Hyper-parameters
    sequence_length = 28
    input_size = 28
    hidden_size = 128
    num_layers = 2
    num_classes = 10
    batch_size = 100
    num_epochs = 2
    learning_rate = 0.003
    
    # MNIST dataset
    train_dataset = torchvision.datasets.MNIST(root='../../data/',
                                               train=True, 
                                               transform=transforms.ToTensor(),
                                               download=True)
    
    test_dataset = torchvision.datasets.MNIST(root='../../data/',
                                              train=False, 
                                              transform=transforms.ToTensor())
    
    # Data loader
    train_loader = torch.utils.data.DataLoader(dataset=train_dataset,
                                               batch_size=batch_size, 
                                               shuffle=True)
    
    test_loader = torch.utils.data.DataLoader(dataset=test_dataset,
                                              batch_size=batch_size, 
                                              shuffle=False)
    
    # Bidirectional recurrent neural network (many-to-one)
    class BiRNN(nn.Module):
        def __init__(self, input_size, hidden_size, num_layers, num_classes):
            super(BiRNN, self).__init__()
            self.hidden_size = hidden_size
            self.num_layers = num_layers
            self.lstm = nn.LSTM(input_size, hidden_size, num_layers, batch_first=True, bidirectional=True)
            self.fc = nn.Linear(hidden_size*2, num_classes)  # 2 for bidirection
        
        def forward(self, x):
            # Set initial states
            h0 = torch.zeros(self.num_layers*2, x.size(0), self.hidden_size).to(device) # 2 for bidirection 
            c0 = torch.zeros(self.num_layers*2, x.size(0), self.hidden_size).to(device)
            
            # Forward propagate LSTM
            out, _ = self.lstm(x, (h0, c0))  # out: tensor of shape (batch_size, seq_length, hidden_size*2)
            
            # Decode the hidden state of the last time step
            out = self.fc(out[:, -1, :])
            return out
    
    model = BiRNN(input_size, hidden_size, num_layers, num_classes).to(device)
    
    
    # Loss and optimizer
    criterion = nn.CrossEntropyLoss()
    optimizer = torch.optim.Adam(model.parameters(), lr=learning_rate)
        
    # Train the model
    total_step = len(train_loader)
    for epoch in range(num_epochs):
        for i, (images, labels) in enumerate(train_loader):
            images = images.reshape(-1, sequence_length, input_size).to(device)
            labels = labels.to(device)
            
            # Forward pass
            outputs = model(images)
            loss = criterion(outputs, labels)
            
            # Backward and optimize
            optimizer.zero_grad()
            loss.backward()
            optimizer.step()
            
            if (i+1) % 100 == 0:
                print ('Epoch [{}/{}], Step [{}/{}], Loss: {:.4f}' 
                       .format(epoch+1, num_epochs, i+1, total_step, loss.item()))
    
    # Test the model
    with torch.no_grad():
        correct = 0
        total = 0
        for images, labels in test_loader:
            images = images.reshape(-1, sequence_length, input_size).to(device)
            labels = labels.to(device)
            outputs = model(images)
            _, predicted = torch.max(outputs.data, 1)
            total += labels.size(0)
            correct += (predicted == labels).sum().item()
    
        print('Test Accuracy of the model on the 10000 test images: {} %'.format(100 * correct / total)) 
    
    # Save the model checkpoint
    torch.save(model.state_dict(), 'model.ckpt')

下面说说LSTM输出后的out的大小。

进入全连接层的out的形状应该是\[sequence\_length, batch, hidden\_size \* num\_directions\]。

但是在代码中，却是out = self.fc(out\[:, -1, :\])，并且注释说是取最后一个时刻的输出，这里就不理解了如果按照上面的说的形状，这里的out\[:, -1, :\]应该是最后一个批次的每个时刻的输出值，但是就和self.fc = nn.Linear(hidden\_size\*2, num\_classes) \# 2 for bidirection产生冲突了。

后面看了别人写的代码，终于想明白了，因为这里定义LSTM时，使用了batch\_first=True这个参数，既然输入是\[batch, sequence\_length, input\_size\],那么输出应该也是这样的排序，所以只要将batch和sequence互换一下就ok了。