Move train hooks from trainer to gan object

[martyn]

Train hooks should be a base level array on the GAN itself. This is because it has functionality that is unrelated to the trainer, such as augmenting discriminator inputs, changing the latent vector, etc.

1.0 configs should continue to work, so it should be something like:

(pseudo-code)

def gan.train_hooks
  search config.train_hooks or config.trainer.train_hooks