import torch
from torch import nn


class GradReverse(torch.autograd.Function):
    """
    Extension of grad reverse layer
    """

    @staticmethod
    def forward(ctx, x, constant):
        ctx.constant = constant
        return x.view_as(x)

    @staticmethod
    def backward(ctx, grad_output):
        grad_output = grad_output.neg() * ctx.constant
        return grad_output, None

    def grad_reverse(x, constant):
        return GradReverse.apply(x, constant)


class DomainCritic(nn.Module):
    def __init__(self):
        super().__init__()
        self.main_module = nn.Sequential(
            nn.Conv2d(in_channels=64, out_channels=128, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(128, affine=True),
            nn.LeakyReLU(0.2),

            nn.Conv2d(in_channels=128, out_channels=256, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(256, affine=True),
            nn.LeakyReLU(0.2),

            nn.Conv2d(in_channels=256, out_channels=512, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(512, affine=True),
            nn.LeakyReLU(0.2),

            nn.Conv2d(in_channels=512, out_channels=1024, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(1024, affine=True),
            nn.LeakyReLU(0.2),
        )

        self.output = nn.Sequential(
            nn.Conv2d(in_channels=1024, out_channels=1, kernel_size=3, stride=1, padding=0, bias=False),
            nn.Sigmoid()
        )

    def forward(self, x, constant):
        x = GradReverse.grad_reverse(x, constant)
        x = self.main_module(x)
        res = self.output(x)
        res = res.view(res.size()[0], -1)
        return res

import torch
from torch import nn


class GradReverse(torch.autograd.Function):
    """
    Extension of grad reverse layer
    """

    @staticmethod
    def forward(ctx, x, constant):
        ctx.constant = constant
        return x.view_as(x)

    @staticmethod
    def backward(ctx, grad_output):
        grad_output = grad_output.neg() * ctx.constant
        return grad_output, None

    def grad_reverse(x, constant):
        return GradReverse.apply(x, constant)


class DomainCritic(nn.Module):
    def __init__(self):
        super().__init__()
        self.main_module = nn.Sequential(
            nn.Conv2d(in_channels=64, out_channels=128, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(128, affine=True),
            nn.LeakyReLU(0.2),

            nn.Conv2d(in_channels=128, out_channels=256, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(256, affine=True),
            nn.LeakyReLU(0.2),

            nn.Conv2d(in_channels=256, out_channels=512, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(512, affine=True),
            nn.LeakyReLU(0.2),

            nn.Conv2d(in_channels=512, out_channels=1024, kernel_size=4, stride=2, padding=1, bias=False),
            nn.InstanceNorm2d(1024, affine=True),
            nn.LeakyReLU(0.2),
        )

        self.output = nn.Sequential(
            nn.Conv2d(in_channels=1024, out_channels=1, kernel_size=3, stride=1, padding=0, bias=False),
            nn.Sigmoid()
        )

    def forward(self, x, constant):
        x = GradReverse.grad_reverse(x, constant)
        x = self.main_module(x)
        res = self.output(x)
        res = res.view(res.size()[0], -1)
        return res

Extension of grad reverse layer