… jsonargparse can correctly parse inputs such as true, false, etc...

before this change jsonargparse gave errors complaining that strings like 'true' or 'false' were given when the annotation expected Optional[Literal[True, False, 'link']]; this is in contrast to most other bool parameters in the ModelCheckpoint callback's constructor. I also added a test in tests/test_pytorch/checkpointing/test_model_checkpoint.py::test_save_last_cli