Uniformize {ModelCheckpoint,Checkpoint}.load_checkpoint

[vfdev-5]

We have to make output uniform between ModelCheckpoint and Checkpoint

ModelCheckpoint.load_checkpoint always can return full filepath. ModelCheckpoint.load_checkpoint can return full filepath if save_handler is DiskSaver, otherwise return only filename (as it is done now)

Also let's fix this error message inside ModelCheckpoint.load_checkpoint

        if not isinstance(self.save_handler, DiskSaver):
            raise RuntimeError(
                f"Unable to save checkpoint, save_handler should be DiskSaver, got {type(self.save_handler)}."
            )

Originally posted by @vfdev-5 in #2461 (comment)