rename loss to losses and update center loss

bob/learn/tensorflow/loss/center_loss.py

bob/learn/tensorflow/losses/BaseLoss.py

bob/learn/tensorflow/losses/ContrastiveLoss.py

bob/learn/tensorflow/losses/StyleLoss.py

bob/learn/tensorflow/losses/TripletLoss.py

bob/learn/tensorflow/losses/__init__.py

bob/learn/tensorflow/losses/center_loss.py

bob/learn/tensorflow/losses/mmd.py

bob/learn/tensorflow/losses/pairwise_confusion.py

bob/learn/tensorflow/losses/pixel_wise.py

bob/learn/tensorflow/losses/utils.py

bob/learn/tensorflow/losses/vat.py