2. UNet

import torch
import torch.nn as nn
from torch.autograd import Variable as V

class Unet(nn.Module):
    def __init__(self):
        super(Unet, self).__init__()
        
        self.down1 = self.conv_stage(3, 8)
        self.down2 = self.conv_stage(8, 16)
        self.down3 = self.conv_stage(16, 32)
        self.down4 = self.conv_stage(32, 64)
        self.down5 = self.conv_stage(64, 128)
        self.down6 = self.conv_stage(128, 256)
        self.down7 = self.conv_stage(256, 512)
        
        self.center = self.conv_stage(512, 1024)
        #self.center_res = self.resblock(1024)
        
        self.up7 = self.conv_stage(1024, 512)
        self.up6 = self.conv_stage(512, 256)
        self.up5 = self.conv_stage(256, 128)
        self.up4 = self.conv_stage(128, 64)
        self.up3 = self.conv_stage(64, 32)
        self.up2 = self.conv_stage(32, 16)
        self.up1 = self.conv_stage(16, 8)
        
        self.trans7 = self.upsample(1024, 512)
        self.trans6 = self.upsample(512, 256)
        self.trans5 = self.upsample(256, 128)
        self.trans4 = self.upsample(128, 64)
        self.trans3 = self.upsample(64, 32)
        self.trans2 = self.upsample(32, 16)
        self.trans1 = self.upsample(16, 8)
        
        self.conv_last = nn.Sequential(
            nn.Conv2d(8, 1, 3, 1, 1),
            nn.Sigmoid()
        )
        
        self.max_pool = nn.MaxPool2d(2)
        
        for m in self.modules():
            if isinstance(m, nn.Conv2d) or isinstance(m, nn.ConvTranspose2d):
                if m.bias is not None:
                    m.bias.data.zero_()

    def conv_stage(self, dim_in, dim_out, kernel_size=3, stride=1, padding=1, bias=True, useBN=False):
        if useBN:
            return nn.Sequential(
              nn.Conv2d(dim_in, dim_out, kernel_size=kernel_size, stride=stride, padding=padding, bias=bias),
              nn.BatchNorm2d(dim_out),
              #nn.LeakyReLU(0.1),
              nn.ReLU(),
              nn.Conv2d(dim_out, dim_out, kernel_size=kernel_size, stride=stride, padding=padding, bias=bias),
              nn.BatchNorm2d(dim_out),
              #nn.LeakyReLU(0.1),
              nn.ReLU(),
            )
        else:
            return nn.Sequential(
              nn.Conv2d(dim_in, dim_out, kernel_size=kernel_size, stride=stride, padding=padding, bias=bias),
              nn.ReLU(),
              nn.Conv2d(dim_out, dim_out, kernel_size=kernel_size, stride=stride, padding=padding, bias=bias),
              nn.ReLU()
            )

    def upsample(self, ch_coarse, ch_fine):
        return nn.Sequential(
            nn.ConvTranspose2d(ch_coarse, ch_fine, 4, 2, 1, bias=False),
            nn.ReLU()
        )
    
    def forward(self, x):
        conv1_out = self.down1(x)
        conv2_out = self.down2(self.max_pool(conv1_out))
        conv3_out = self.down3(self.max_pool(conv2_out))
        conv4_out = self.down4(self.max_pool(conv3_out))
        conv5_out = self.down5(self.max_pool(conv4_out))
        conv6_out = self.down6(self.max_pool(conv5_out))
        conv7_out = self.down7(self.max_pool(conv6_out))
        
        out = self.center(self.max_pool(conv7_out))
        #out = self.center_res(out)

        out = self.up7(torch.cat((self.trans7(out), conv7_out), 1))
        out = self.up6(torch.cat((self.trans6(out), conv6_out), 1))
        out = self.up5(torch.cat((self.trans5(out), conv5_out), 1))
        out = self.up4(torch.cat((self.trans4(out), conv4_out), 1))
        out = self.up3(torch.cat((self.trans3(out), conv3_out), 1))
        out = self.up2(torch.cat((self.trans2(out), conv2_out), 1))
        out = self.up1(torch.cat((self.trans1(out), conv1_out), 1))

        out = self.conv_last(out)

        return out

bilibili有up一步一步教写改良后的，【图像分割UNet硬核讲解(带你手撸unet代码)】https://www.bilibili.com/video/BV11341127iK