Is it possible to make `validation_step` and `val_dataloader` no-ops?
Is your feature request related to a problem? Please describe.
Sometimes I don't have a separate validation split, only a train/test split.  I'm trying out pytorch-lightning to prototype / experiment, and trying to see what the best of way of doing this is.
I could make the train dataset and then do torch.utils.data.random_split or use torch.utils.data.SubsetRandomSampler to build a validation set as well, but if I don't have enough data (or just don't want to do a separate validation step) this isn't ideal.
Describe the solution you'd like
I'd like to be able to implement only the training_step, train_dataloader, and test_dataloader methods and then have the validation step and validation metrics be omitted (maybe explicit no-ops).  Right now, I'm experimenting with having an empty DataLoader for the validation data.
Describe alternatives you've considered

Implement val_dataloader with an empty (dummy) DataLoader

Not sure if this will work yet (if lightning will still call validation_step and validation_end).