Update Discriminators Strtegy

[BOBrown]

Thank you very much for your contribution.
The author says in the original paper that "update the discriminators using a history of generated images rather than the ones produced by the latest generators". I read your code carefully and found that there seems to be no updating Discriminator using historically generated images.

[vanhuyz]

Actually I did. Generated images are pushed into a pool and used to update discriminators later.

https://github.com/vanhuyz/CycleGAN-TensorFlow/blob/master/train.py#L97-L98
https://github.com/vanhuyz/CycleGAN-TensorFlow/blob/master/utils.py#L33