PyTorch 介绍 | 快速开始

本节介绍有关机器学习常见任务重的API。请参阅每一节的链接以深入了解。

Working with data

PyTorch有两个有关数据工作的原型： torch.utils.data.DataLoader 和 torch.utils.data.Dataset。 Dataset 存储了样本及其对应的标签，而 DataLoader为 Dataset 生成了一个迭代器。

import torch
from torch import nn
from torch.utils.data import DataLoader
from torchvision import datasets
from torchvision.transforms import ToTensor, Lambda, Compose
import matplotlib.pyplot as plt

torchvision.datasets 模块包含多种真实世界的视觉数据集 Dataset 对象，如CIFAR、COCO（full list here）。本教程中，我们使用FashionMNIST数据集。每个TorchVision Dataset 均包括两个参数： transform 和 target_transform分别用于修改样本和标签。

从公开数据集上下载训练数据
training_data = datasets.FashionMNIST(
    root='data',
    train=True,
    download=True,
    transform=ToTensor(),
)

Download test data from open datasets.

test_data = datasets.FashionMNIST(
    root='data',
    train=False,
    download=True,
    transform=ToTensor(),
)

输出：

Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/train-images-idx3-ubyte.gz
Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/train-images-idx3-ubyte.gz to data/FashionMNIST/raw/train-images-idx3-ubyte.gz
Extracting data/FashionMNIST/raw/train-images-idx3-ubyte.gz to data/FashionMNIST/raw

Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/train-labels-idx1-ubyte.gz
Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/train-labels-idx1-ubyte.gz to data/FashionMNIST/raw/train-labels-idx1-ubyte.gz
Extracting data/FashionMNIST/raw/train-labels-idx1-ubyte.gz to data/FashionMNIST/raw

Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/t10k-images-idx3-ubyte.gz
Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/t10k-images-idx3-ubyte.gz to data/FashionMNIST/raw/t10k-images-idx3-ubyte.gz
Extracting data/FashionMNIST/raw/t10k-images-idx3-ubyte.gz to data/FashionMNIST/raw

Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/t10k-labels-idx1-ubyte.gz
Downloading http://fashion-mnist.s3-website.eu-central-1.amazonaws.com/t10k-labels-idx1-ubyte.gz to data/FashionMNIST/raw/t10k-labels-idx1-ubyte.gz
Extracting data/FashionMNIST/raw/t10k-labels-idx1-ubyte.gz to data/FashionMNIST/raw

我们将 Dataset 作为参数传递给 DataLoader。这为我们的dataset包装了一个迭代器，并支持自动生成batch、抽样、打乱和多进程数据加载。这里定义了一个大小为64的batch，即，dataloader迭代的每一个元素将返回一个包含64个样本及对应标签的batch。

batch_size = 64

Create data loaders
train_dataloader = DataLoader(training_data, batch_size=batch_size)
test_dataloader = DataLoader(test_data, batch_size=batch_size)

for X, y in test_dataloader:
    print("Shape of X [N, C, H, W]: ", X.shape)
    print("Shape of y: ", y.shape, y.dtpe)
    break

输出：

Shape of X [N, C, H, W]:  torch.Size([64, 1, 28, 28])
Shape of y:  torch.Size([64]) torch.int64

创建模型

为了在PyTorch定义模型，我们创建了一个类，继承自nn.Module。我们在 __init__函数中定义是网络的layers，并在 forward 函数中指定data如何通过网络。为加快神经网络中的操作，若GPU可用，则把其移动到GPU上。

Get cpu or gpu device for training
device = 'cuda' if torch.cuda.is_availabel() else "cpu"
print(f"Using {device} device")

Define model
class NeuralNetwork(nn.Module):
    def __init__(self):
        super(NeuralNetwork, self).__init__()
        self.flatten = nn.Flatten()
        self.linear_relu_stack = nn.Sequential(
            nn.Linear(28*28, 512),
            nn.ReLU(),
            nn.Linear(512, 512),
            nn.ReLU(),
            nn.Linear(512, 10)

        )

    def forward(self, x):
        x = self.flatten(x)
        logits = self.linear_relu_stack(x)
        return logits

model = NeuralNetwork().to(device)
print(model)

输出：

Using cuda device
NeuralNetwork(
  (flatten): Flatten(start_dim=1, end_dim=-1)
  (linear_relu_stack): Sequential(
    (0): Linear(in_features=784, out_features=512, bias=True)
    (1): ReLU()
    (2): Linear(in_features=512, out_features=512, bias=True)
    (3): ReLU()
    (4): Linear(in_features=512, out_features=10, bias=True)
  )
)

优化模型参数

loss_fn = nn.CrossEntropyLoss()
optimizer = torch.optim.SGD(model.parameters(), lr=1e-3)

在单个训练循环中，模型在训练集上作出预测（分批喂给模型），并且反向传播预测误差来调整模型参数。

def train(dataloader, model, loss_fn, optimizer):
    size = len(dataloader.dataset)
    model.train()
    for batch, (X, y) in enumerate(dataloader):
        X, y = X.to(device), y.to(device)

        # Compute prediction error
        pred = model(X)
        loss = loss_fn(pred, y)

        # Backpropagation
        optimizer.zero_grad()
        loss.backward()
        optimizer.step()

        if batch % 100 == 0:
            loss, current = loss.item(), batch * len(x)
            print(f"loss: {loss:>7f} [{current:>5d}/{size:>5d}]")

我们还可以检查模型在测试集上的性能，确保模型是在学习。

def test(dataloader, model, loss_fn):
    size = len(dataloader.dataset)
    num_batches = len(dataloader)
    model.eval()
    test_loss, correct = 0, 0
    with torch.no_grad():
        for X, y in dataloader:
            X, y = X.to(device), y.to(device)
            pred = model(X)
            test_loss += loss_fn(pred, y).item()
            # 第一维度是batch，第二维度是预测值
            correct += (pred.argmax(1) == y).type(torch.float).sum().item()
    test_loss /= num_batches
    correct /= size
    print(f"Test Error: \n Accuracy: {(100 * correct):>0.1f}%, Avg loss: {test_loss:>8f}\n")

训练过程由几次迭代（ epochs）组成。每一个epoch，模型学习参数，作出更好的预测。我们在每次epoch都打印了模型的准确率和损失，我们希望看到随着每次epoch，准确率升高，而损失降低。

epochs = 5
for t in range(epochs):
    print(f"Epoch {t+1}\n-----------------------")
    train(train_dataloader, model, loss_fn, optimizer)
    test(test_dataloader, model, loss_fn)
print("Done!")

输出：

点击查看代码

`
Epoch 1
loss: 2.169147 [ 0/60000]
loss: 2.165468 [ 6400/60000]
loss: 2.118014 [12800/60000]
loss: 2.129221 [19200/60000]
loss: 2.074899 [25600/60000]
loss: 2.022606 [32000/60000]
loss: 2.033795 [38400/60000]
loss: 1.976709 [44800/60000]
loss: 1.982757 [51200/60000]
loss: 1.881978 [57600/60000]
Test Error:
Accuracy: 57.4%, Avg loss: 1.902724

Epoch 3
loss: 1.592845 [ 0/60000]
loss: 1.556097 [ 6400/60000]
loss: 1.417763 [12800/60000]
loss: 1.478243 [19200/60000]
loss: 1.357680 [25600/60000]
loss: 1.356057 [32000/60000]
loss: 1.360733 [38400/60000]
loss: 1.298324 [44800/60000]
loss: 1.329920 [51200/60000]
loss: 1.219030 [57600/60000]
Test Error:
Accuracy: 63.4%, Avg loss: 1.250318

Epoch 5

Original: https://www.cnblogs.com/DeepRS/p/15727075.html
Author: Deep_RS
Title: PyTorch 介绍 | 快速开始