Adds option to disable post train permanent checkpoint

[epwalsh]

With this you can configure the checkpointer callback to only save
ephemeral checkpoints, and leave nothing at the end of the run by setting:

--trainer.callbacks.checkpointer.pre_train_checkpoint=false
--trainer.callbacks.checkpointer.post_train_checkpoint=false
--trainer.callbacks.checkpointer.save_interval=null
--trainer.callbacks.checkpointer.ephemeral_save_interval=X