8.1 损失函数
① Loss损失函数一方面计算实际输出和目标之间的差距。
② Loss损失函数另一方面为我们更新输出提供一定的依据。
8.2 L1loss损失函数
① L1loss数学公式如下图所示，例子如下下图所示。
import torch
from torch.nn import L1Loss
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() # 默认为 maen
result = loss(inputs,targets)
print(result)结果：
tensor(0.6667)
import torch
from torch.nn import L1Loss
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(reduction='sum') # 修改为sum，三个值的差值，然后取和
result = loss(inputs,targets)
print(result)结果：
tensor(2.)
8.3 MSE损失函数
① MSE损失函数数学公式如下图所示。
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_mse = nn.MSELoss()
result_mse = loss_mse(inputs,targets)
print(result_mse)结果：
tensor(1.3333)
8.4 交叉熵损失函数
① 交叉熵损失函数数学公式如下图所示。
import torch
from torch.nn import L1Loss
from torch import nnx = torch.tensor([0.1,0.2,0.3])
y = torch.tensor([1])
x = torch.reshape(x,(1,3)) # 1的 batch_size，有三类
loss_cross = nn.CrossEntropyLoss()
result_cross = loss_cross(x,y)
print(result_cross)结果：
tensor(1.1019)
8.5 搭建神经网络
import torch
import torchvision
from torch import nn
from torch.nn import Conv2d, MaxPool2d, Flatten, Linear, Sequential
from torch.utils.data import DataLoader
from torch.utils.tensorboard import SummaryWriterdataset = torchvision.datasets.CIFAR10("./dataset",train=False,transform=torchvision.transforms.ToTensor(),download=True)
dataloader = DataLoader(dataset, batch_size=1,drop_last=True)class Tudui(nn.Module):def __init__(self):super(Tudui, 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 xtudui = Tudui()
for data in dataloader:imgs, targets = dataoutputs = tudui(imgs)print(outputs)print(targets)结果：
8.6 数据集计算损失函数
import torch
import torchvision
from torch import nn
from torch.nn import Conv2d, MaxPool2d, Flatten, Linear, Sequential
from torch.utils.data import DataLoader
from torch.utils.tensorboard import SummaryWriterdataset = torchvision.datasets.CIFAR10("./dataset",train=False,transform=torchvision.transforms.ToTensor(),download=True)
dataloader = DataLoader(dataset, batch_size=64,drop_last=True)class Tudui(nn.Module):def __init__(self):super(Tudui, 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 xloss = nn.CrossEntropyLoss() # 交叉熵
tudui = Tudui()
for data in dataloader:imgs, targets = dataoutputs = tudui(imgs)result_loss = loss(outputs, targets) # 计算实际输出与目标输出的差距print(result_loss)结果：
8.7 损失函数反向传播
① 反向传播通过梯度来更新参数，使得loss损失最小，如下图所示。
import torch
import torchvision
from torch import nn
from torch.nn import Conv2d, MaxPool2d, Flatten, Linear, Sequential
from torch.utils.data import DataLoader
from torch.utils.tensorboard import SummaryWriterdataset = torchvision.datasets.CIFAR10("./dataset",train=False,transform=torchvision.transforms.ToTensor(),download=True)
dataloader = DataLoader(dataset, batch_size=64,drop_last=True)class Tudui(nn.Module):def __init__(self):super(Tudui, 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 xloss = nn.CrossEntropyLoss() # 交叉熵
tudui = Tudui()
for data in dataloader:imgs, targets = dataoutputs = tudui(imgs)result_loss = loss(outputs, targets) # 计算实际输出与目标输出的差距result_loss.backward() # 计算出来的 loss 值有 backward 方法属性，反向传播来计算每个节点的更新的参数。这里查看网络的属性 grad 梯度属性刚开始没有，反向传播计算出来后才有，后面优化器会利用梯度优化网络参数。 print("ok")
