Customize multiple dataloaders behaviour in train vs val/test
User should be able to determine if multiple dataloaders will run simultaneously (passed as a list/dict), or get called sequentially.
This can be done with a trainer flag with multiple options.
Background:
Currently, in training step you get the different batches at the same time as list or dict, but for validation step it gets called multiple times with a dataloader_idx. Just thought it would be nicer if how multiple dataloaders are handled is consistent across training, val and test.