metrics all over the place in GAN implementations

see if it can be improved so that is general and that it is a single call, preferably in BaseGAN, and takes care if it's a cycle-consistency model or not and so on.

rn the problem is that the calculation of metrics is located in different places in GANs (see CycleGAN, RevGAN), which makes it not very readable and a pain to remember and place properly in new GAN implementations.