Model checkpoint naming consistency

[carmocca]

🚀 Feature

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

Motivation

1. 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 -.
2. 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

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

[Borda]

that sounds good to me, mind link some reference for this "standard way"? also sending a PR is very welcome 🐰
cc: @PyTorchLightning/core-contributors

[carmocca]

I couldn't find any good reference other than Python's use of underscore in variable names (see PEP8). I will make "-" (default field splitter) a modifiable variable.