Validation step is skipped when on_train_batch_start returns -1
🐛 Bug
It seems that when on_train_batch_start returns -1 the validation step is skipped.
Expected behavior
The trainer should skip the training epoch and start the evaluation.
 - pytorch-lightning version 1.0.4
 - PyTorch Version (e.g., 1.0): 1.7
 - OS (e.g., Linux): Linux
 - How you installed PyTorch (`conda`, `pip`, source): conda
 - Python version: 3.7