Model checkpoint naming consistency
🚀 Feature

Modify str_ver inside format_checkpoin_name()
Let ModelCheckpoint.CHECKPOINT_NAME_LAST contain templates.

Motivation

Going by format_checkpoint_name's docstring example. It is standard to use - to split metrics and _ to split metric names. This makes str_ver's format inconsistent by using -.
It would be helpful to be able to set ModelCheckpoint.CHECKPOINT_NAME_LAST='last-{epoch}-{acc.3f}.ckpt' with the same parsing behaviour as regular checkpoints.

Pitch

Replace f"_v{ver}" for f"-v{ver}" or f".{ver}"
Parse ModelCheckpoint.CHECKPOINT_NAME_LAST with format_checkpoint_name()