Critic to train a `GAN`.

def gan_critic(n_channels:int=3, nf:int=128, n_blocks:int=3, p:int=0.15):
    "Critic to train a `GAN`."
    layers = [
        _conv(n_channels, nf, ks=4, stride=2),
        nn.Dropout2d(p/2),
        res_block(nf, dense=True,**_conv_args)]
    nf *= 2 # after dense block
    for i in range(n_blocks):
        layers += [
            nn.Dropout2d(p),
            _conv(nf, nf*2, ks=4, stride=2, self_attention=(i==0))]
        nf *= 2
    layers += [
        _conv(nf, 1, ks=4, bias=False, padding=0, use_activ=False),
        Flatten()]
    return nn.Sequential(*layers)