8-pytorch-损失函数与反向传播

b站小土堆pytorch教程学习笔记
根据loss更新模型参数

1.计算实际输出与目标之间的差距

2.为我们更新输出提供一定的依据（反向传播）

1 MSEloss

import torch
from torch.nn import L1Loss
from torch import nn

inputs=torch.tensor([1,2,3],dtype=torch.float32)
targets=torch.tensor([1,2,5],dtype=torch.float32)

inputs=torch.reshape(inputs,(-1,1,1,3))
targets=torch.reshape(targets,(-1,1,1,3))

loss=L1Loss()
result=loss(inputs,targets)

loss_mse=nn.MSELoss()
result_mse=loss_mse(inputs,targets)

print(result)
print(result_mse)

tensor(0.6667)
tensor(1.3333)

2 Cross EntropyLoss

x=torch.tensor([0.1,0.2,0.3])#需要reshape为要求的(batch_size,class)
y=torch.tensor([1])#target已经为要求的batch_size无需reshape
x=torch.reshape(x,(-1,3))
loss_cross=nn.CrossEntropyLoss()
result_cross=loss_cross(x,y)
print(result_cross)

tensor(1.1019)

3 在具体的神经网络中使用loss

import torch
import torchvision.datasets
from torch import nn
from torch.nn import Conv2d, MaxPool2d, Flatten, Linear, Sequential
from torch.utils.data import DataLoader
from torch.utils.tensorboard import SummaryWriter

dataset=torchvision.datasets.CIFAR10('dataset',train=False,
                                     transform=torchvision.transforms.ToTensor(),
                                     download=True)
dataloader=DataLoader(dataset,batch_size=1)

class Han(nn.Module):
    def __init__(self):
        super(Han, self).__init__()
        self.model1=Sequential(
            Conv2d(3,32,5,padding=2),
            MaxPool2d(2),
            Conv2d(32,32,5,padding=2),
            MaxPool2d(2),
            Conv2d(32,64,5,padding=2),
            MaxPool2d(2),
            Flatten(),
            Linear(1024,64),
            Linear(64,10)
        )

    def forward(self,x):
        x=self.model1(x)
        return x

loss=nn.CrossEntropyLoss()
han=Han()
for data in dataloader:
    imgs,target=data
    output=han(imgs)
    # print(target)
    # print(output)
    result_loss=loss(output,target)
    print(result_loss)

*tensor(7)

tensor(\[ 0.0057, -0.0201, -0.0796, 0.0556, -0.0625, 0.0125, -0.0413, -0.0056, *0.0624, -0.1072], grad_fn=)...*

tensor(2.2664, grad_fn=)...

4 反向传播 优化器

1. 定义优化器
2. 将待更新的每个参数梯度清零
3. 调用损失函数的反向传播函数求出每个节点的梯度
4. 使用step函数对模型的每个参数调优

import torch
import torchvision.datasets
from torch import nn
from torch.nn import Conv2d, MaxPool2d, Flatten, Linear, Sequential
from torch.utils.data import DataLoader
from torch.utils.tensorboard import SummaryWriter

dataset=torchvision.datasets.CIFAR10('dataset',train=False,
                                     transform=torchvision.transforms.ToTensor(),
                                     download=True)
dataloader=DataLoader(dataset,batch_size=64)

class Han(nn.Module):
    def __init__(self):
        super(Han, self).__init__()
        self.model1=Sequential(
            Conv2d(3,32,5,padding=2),
            MaxPool2d(2),
            Conv2d(32,32,5,padding=2),
            MaxPool2d(2),
            Conv2d(32,64,5,padding=2),
            MaxPool2d(2),
            Flatten(),
            Linear(1024,64),
            Linear(64,10)
        )

    def forward(self,x):
        x=self.model1(x)
        return x

loss=nn.CrossEntropyLoss()
han=Han()
optim=torch.optim.SGD(han.parameters(),lr=0.01)

for epoch in range(5):
    running_loss=0.0#一个epoch结束的loss和
    for data in dataloader:
        imgs,target=data
        output=han(imgs)

        result_loss=loss(output,target)#每次迭代的loss
        optim.zero_grad()#将网络中每个可调节参数对应的梯度调为0
        result_loss.backward()#优化器需要每个参数的梯度，使用反向传播获得
        optim.step()#对每个参数调优
        running_loss=running_loss+result_loss
    print(running_loss)

Files already downloaded and verified
tensor(361.0316, grad_fn=)
tensor(357.6938, grad_fn=)
tensor(343.0560, grad_fn=)
tensor(321.8132, grad_fn=)
tensor(313.3173, grad_fn=)