Feat discriminator unet

[wr0124]

https://arxiv.org/abs/2002.12655

This PR implements the pixel et bottleneck discriminator.

example :

python3 train.py --dataroot /path/to/horse2zebra --checkpoints_dir /path/to/checkpoints --name horse2zebra --config_json examples/example_gan_horse2zebra.json --D_netDs projected_d unet_128_d basic