UNET self.ups alternative in forward pass

[gaffinetB]

Implemented alternative way to make skip connections work in forward pass for UNET model.

Instead of iterating with a step of 2 over self.ups, iterate over the reversed(skip_connections). The indices are adapted accordingly.

Changes come as a reaction to PyTorch Image Segmentation Tutorial with U-NET: everything from scratch baby at 16'00'' (love the video)

Hope it helps. Hope it's more intuitive as approach.