2012年提出的网络结构,获得了当年的ImageNet LSVRC的冠军。
AlexNet在激活函数上选取了非线性非饱和的relu函数,在训练阶段梯度衰减快慢方面,relu函数比传统神经网络所选取的非线性饱和函数(如sigmoid函数,tanh函数)要快许多。
AlexNet在双gpu上运行,每个gpu负责一半网络的运算
采用局部响应归一化(LRN)。对于非饱和函数relu来说,不需要对其输入进行标准化,但Alex等人发现,在relu层加入LRN,可形成某种形式的横向抑制,从而提高网络的泛华能力。
池化方式采用overlapping pooling。即池化窗口的大小大于步长,使得每次池化都有重叠的部分。(ps:这种重叠的池化方式比传统无重叠的池化方式有着更好的效果,且可以避免过拟合现象的发生)
import torch.nn as nn
import torch
from torchinfo import summary
class AlexNet(nn.Module):
def __init__(self, num_classes=1000, init_weights=False):
super(AlexNet, self).__init__()
self.features = nn.Sequential( #打包
nn.Conv2d(3, 48, kernel_size=11, stride=4, padding=2), # input[3, 224, 224] output[48, 55, 55] 自动舍去小数点后
nn.ReLU(inplace=True), #inplace 可以载入更大模型
nn.MaxPool2d(kernel_size=3, stride=2), # output[48, 27, 27] kernel_num为原论文一半
nn.Conv2d(48, 128, kernel_size=5, padding=2), # output[128, 27, 27]
nn.ReLU(inplace=True),
nn.MaxPool2d(kernel_size=3, stride=2), # output[128, 13, 13]
nn.Conv2d(128, 192, kernel_size=3, padding=1), # output[192, 13, 13]
nn.ReLU(inplace=True),
nn.Conv2d(192, 192, kernel_size=3, padding=1), # output[192, 13, 13]
nn.ReLU(inplace=True),
nn.Conv2d(192, 128, kernel_size=3, padding=1), # output[128, 13, 13]
nn.ReLU(inplace=True),
nn.MaxPool2d(kernel_size=3, stride=2), # output[128, 6, 6]
)
self.classifier = nn.Sequential(
nn.Dropout(p=0.5),
nn.Linear(128 * 6 * 6, 2048),
nn.ReLU(inplace=True),
nn.Dropout(p=0.5),
nn.Linear(2048, 2048),
nn.ReLU(inplace=True),
nn.Linear(2048, num_classes),
)
if init_weights:
self._initialize_weights()
def forward(self, x):
x = self.features(x)
x = torch.flatten(x, start_dim=1) #展平 或者view()
x = self.classifier(x)
return x
def _initialize_weights(self):
for m in self.modules():
if isinstance(m, nn.Conv2d):
nn.init.kaiming_normal_(m.weight, mode='fan_out', nonlinearity='relu') #何教授方法
if m.bias is not None:
nn.init.constant_(m.bias, 0)
elif isinstance(m, nn.Linear):
nn.init.normal_(m.weight, 0, 0.01) #正态分布赋值
nn.init.constant_(m.bias, 0)
if __name__=='__main__':
model=AlexNet()
x=torch.randn(1,3,224,224)
summary(model,x.shape)