如何用pytorch在fashionmnist数据集中使把数据分为训练集，测试集和验证集，在使用dropout方法

首先，我们需要导入PyTorch和FashionMNIST数据集：

import torch
import torchvision
import torchvision.transforms as transforms

transform = transforms.Compose(
    [transforms.ToTensor(),
     transforms.Normalize((0.5,), (0.5,))])

trainset = torchvision.datasets.FashionMNIST(root='./data', train=True,
                                        download=True, transform=transform)
testset = torchvision.datasets.FashionMNIST(root='./data', train=False,
                                       download=True, transform=transform)

接下来，我们可以使用`random_split`方法将训练集分为训练集和验证集：

trainset, valset = torch.utils.data.random_split(trainset, [50000, 10000])

然后，我们可以使用`DataLoader`来加载数据集：

trainloader = torch.utils.data.DataLoader(trainset, batch_size=64,
                                          shuffle=True, num_workers=2)
valloader = torch.utils.data.DataLoader(valset, batch_size=64,
                                          shuffle=True, num_workers=2)
testloader = torch.utils.data.DataLoader(testset, batch_size=64,
                                         shuffle=False, num_workers=2)

接下来，我们可以定义我们的神经网络模型，并在其中使用`nn.Dropout`方法来使用dropout：

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(1, 6, 5)
        self.pool = nn.MaxPool2d(2, 2)
        self.conv2 = nn.Conv2d(6, 16, 5)
        self.fc1 = nn.Linear(16 * 4 * 4, 120)
        self.fc2 = nn.Linear(120, 84)
        self.fc3 = nn.Linear(84, 10)
        self.dropout = nn.Dropout(p=0.5)

    def forward(self, x):
        x = self.pool(F.relu(self.conv1(x)))
        x = self.pool(F.relu(self.conv2(x)))
        x = x.view(-1, 16 * 4 * 4)
        x = F.relu(self.fc1(x))
        x = self.dropout(x)
        x = F.relu(self.fc2(x))
        x = self.dropout(x)
        x = self.fc3(x)
        return x

net = Net()

最后，我们可以定义我们的优化器和损失函数，并开始训练我们的模型：

import torch.optim as optim

criterion = nn.CrossEntropyLoss()
optimizer = optim.SGD(net.parameters(), lr=0.001, momentum=0.9)

for epoch in range(10):  # 训练网络10个epoch

    running_loss = 0.0
    for i, data in enumerate(trainloader, 0):
        inputs, labels = data

        optimizer.zero_grad()

        outputs = net(inputs)
        loss = criterion(outputs, labels)
        loss.backward()
        optimizer.step()

        running_loss += loss.item()

    # 每个epoch之后，在验证集上测试模型
    correct = 0
    total = 0
    with torch.no_grad():
        for data in valloader:
            images, labels = data
            outputs = net(images)
            _, predicted = torch.max(outputs.data, 1)
            total += labels.size(0)
            correct += (predicted == labels).sum().item()

    print('Epoch %d: Loss: %.3f Validation Accuracy: %.3f' %
          (epoch + 1, running_loss / len(trainloader),
           100 * correct / total))

print('Finished Training')