Add validate every n epochs

[alexriedel1]

Hey, as an enhancement for custom model training, I propose to add a configuration argument so that the trainer does not evaluate after every epoch but a specification to validate every n epochs.

This would speed up the training process as the validation on adversarial examples can take quite a long time and maybe doesn't make sense in the early stages of a training.

[tamltlkdn]

Hello, why do we need to validate the adversarial examples during training? Are we using/reporting the best model instead of the last model??

[alexriedel1]

We want to know if the model is improving (learning) during the training

[tamltlkdn]

I verified by myself and find that the PGD 7 ACC reaches its peak after the lr is dropped (probably at 100 epoch or 150 epoch). The final (at 200 epoch) model achieves a lower PGD 20 than the best model. I load the pre-trained model ResNet50 provided in this repo and it said the epoch is 152. It looks like they provide the best one, not the last model.