Adding fid calculation at the end of epochs (at train.py)

[OmriKaduri]

Hi,

Thanks for a great repo for experimenting with CycleGAN and pix2pix!

I wanted to compute FID scores during training (between the validation set and the generated images on validation), so I added this as an option at train.py.

Let me know if any changes needed. I believe it might be useful for others.

[rickkk856]

While we still don't have the implemented metrics I prepared simple way to get FID after training using test data real images on domain B and generated images from domain B --> input domain is simply ignored

https://github.com/rickkk856/Batch_FID_PyTorchPix2Pix

[junyanz]

We cannot accept this PR at this time, due to two reasons:

1. this PR (training time FID) may introduce too many changes to train.py. To add metrics to the training code, we need to add a metric abstract base class metric.py that can work for different metrics, implement an FID metric class metric_FID.py, and keep the training code simple.
2. Pytorch-FID also has anti-aliasing issues. FID calculation is quite complicated as it also involves which reference dataset you are using and how many samples you are using. Please see the clean-fid project for more details.

It might be great to first add the FID evaluation code for pre-trained checkpoints (after training is done). We will see if we could do that in the near future.