format_checkpoint_name takes global step
🚀 Feature
Since there is val_check_interval, the model checkpointer could also be enhanced to save in the form of, for example checkpoint_iter2000.ckpt, by defining
checkpoint_callback = ModelCheckpoint(filepath='checkpoint_iter{global_step}')
https://github.com/PyTorchLightning/PyTorch-Lightning/blob/master/pytorch_lightning/callbacks/model_checkpoint.py#L191