PyTorch—— softmax 的从零开始实现

快来打我* 2022-12-04 01:02 220阅读 0赞

### PyTorch—— softmax 的从零开始实现 ###

*   *  一、获取数据
     *  二、初始化模型参数
     *  三、实现softmax运算
     *  四、定义模型
     *  五、定义损失函数
     *  六、计算分类准确率
     *  七、定义优化函数
     *  八、训练模型

本文是学习[《动手学深度学习（pytorch）》“3.6 softmax 的从零开始实现”][pytorch_3.6 softmax] 的笔记，具体解释请参考原文。

#导入包
    import torch
    import torchvision
    import torch.utils.data as Data

## 一、获取数据 ##

参考 [PyTorch—— 图像分类数据集（Fashion-MNIST）][PyTorch_ _Fashion-MNIST]，把“一、二”两部分整合起来写成一个函数：

def load_data_fashion_mnist(batch_size):
    	mnist_train = torchvision.datasets.FashionMNIST(root='~/Datasets/FashionMNIST', train=True, download=True, transform=torchvision.transforms.ToTensor())
    	mnist_test = torchvision.datasets.FashionMNIST(root='~/Datasets/FashionMNIST', train=False, download=True, transform=torchvision.transforms.ToTensor())
    
    	train_iter = Data.DataLoader(dataset=mnist_train, batch_size=batch_size, shuffle=True)
    	test_iter = Data.DataLoader(dataset=mnist_test, batch_size=batch_size, shuffle=True)
    	return train_iter, test_iter

batch_size = 256
    train_iter, test_iter = load_data_fashion_mnist(batch_size)

## 二、初始化模型参数 ##

需要模型参数梯度

num_inputs = 784
    num_outputs = 10
    
    weights = torch.normal(0, 1, (num_inputs, num_outputs))
    bias = torch.zeros(num_outputs)
    
    weights.requires_grad_(requires_grad=True)
    bias.requires_grad_(requires_grad=True)

## 三、实现softmax运算 ##

[softmax运算原理介绍][softmax]

def softmax(X):
    	X_exp = X.exp()
    	partition = X_exp.sum(dim=1, keepdim=True)
    	return X_exp / partition

## 四、定义模型 ##

def net(X):
    	return softmax(torch.mm(X.view(-1, num_inputs), weights) + bias)

## 五、定义损失函数 ##

损失函数使用交叉熵函数。`gather`函数的第一个参数是维度。

def cross_entropy(y, y_hat):
    	return -torch.log(y_hat.gather(1, y.view(-1,1)))

## 六、计算分类准确率 ##

**1、首先是训练集的分类准确率计算函数**，只用计算 train\_iter 中当前 batch\_size 大小的数据的分类准确率即可。

def accuracy(y, y_hat):
    	return (y_hat.argmax(dim=1)==y).float().mean().item()

**2、第二个是测试集的分类准确率计算函数**，需要计算 test\_iter 中所有数据的分类准确率。

def evaluate_accuracy(data_iter, net):
    	acc_sum, num = 0.0, 0
    	for X, y in data_iter:
    		acc_sum += (net(X).argmax(dim=1)==y).float().sum().item()
    		num += y.shape[0]
    	return acc_sum / num

## 七、定义优化函数 ##

def sgd(params, batch_size, lr):
    	for param in params:
    		param.data -= lr * param.grad / batch_size

## 八、训练模型 ##

num_epochs = 5
    lr = 0.5
    
    for epoch in range(num_epochs):
    	acc_sum, l_sum, num = 0.0, 0.0, 0
    	for X, y in train_iter:
    		y_hat = net(X)
    		l = cross_entropy(y, y_hat).sum()
    		l_sum += l
    		l.backward()
    
    		sgd([weights, bias], batch_size, lr)
    		weights.grad.data.zero_()
    		bias.grad.data.zero_()
    		
    		acc_sum += accuracy(y, y_hat)
    		num += y.shape[0]
    	test_acc = evaluate_accuracy(test_iter, net)
    	print('Step:%d, Loss:%.3f, train accuracy:%.3f, test accuracy:%.3f' % (epoch+1, l_sum/num, acc_sum/num, test_acc))

我们把训练模型封装成一个函数，由外界决定：

*  **优化函数**是自定义 还是 使用pytorch中自带的函数
 *  **损失函数**是自定义 还是 使用pytorch中自带的函数

def train_softmax(net, train_iter, test_iter, loss, num_epochs, batch_size, params=None, lr=None, optimizer=None):
    	for epoch in range(num_epochs):
    		acc_sum, l_sum, num = 0.0, 0.0, 0
    		for X, y in train_iter:
    			y_hat = net(X)
    			l = loss(y, y_hat).sum()
    			l_sum += l#.item()
    	
    			if optimizer is not None:
    				optimizer.zero_grad()
    			elif params is not None and params[0].grad is not None:
    				for param in params:
    					param.grad.data.zero_()
    			l.backward()
    			if optimizer is not None:
    				optimizer.step()
    			else:
    				sgd(params, batch_size, lr)
    			
    			acc_sum += accuracy(y, y_hat)
    			num += y.shape[0]
    		test_acc = evaluate_accuracy(test_iter, net)
    		print('Step:%d, Loss:%.3f, train accuracy:%.3f, test accuracy:%.3f' % (epoch+1, l_sum/num, acc_sum/num, test_acc))

调用：

train_softmax(net, train_iter, test_iter, cross_entropy, num_epochs, batch_size, params=[weights, bias], lr=lr)
    
    #Step:1, Loss:2.370, train accuracy:0.002, test accuracy:0.713
    #Step:2, Loss:1.362, train accuracy:0.003, test accuracy:0.761
    #Step:3, Loss:1.164, train accuracy:0.003, test accuracy:0.779
    #Step:4, Loss:1.077, train accuracy:0.003, test accuracy:0.789
    #Step:5, Loss:1.012, train accuracy:0.003, test accuracy:0.789

[pytorch_3.6 softmax]: https://tangshusen.me/Dive-into-DL-PyTorch/#/chapter03_DL-basics/3.6_softmax-regression-scratch
[PyTorch_ _Fashion-MNIST]: https://blog.csdn.net/dear_jing/article/details/108405050
[softmax]: https://tangshusen.me/Dive-into-DL-PyTorch/#/chapter03_DL-basics/3.4_softmax-regression