Removing the last unnecessary downsampling in Discriminator

[stachu86]

Refactoring of forward() method in Discriminator, to prevent last (unnecessary) downsampling.

    def forward(self, x):
        results = []
        for key, disc in self.model.items():
            results.append(disc(x))
            x = self.downsample(x) # <-- this is unnecessary in the last iteration of the loop
        return results