Fix bug. The parameter --keep_last_n_checkpoints -1 doesn't work.

[shizhengLi]

Modify logic of clean_last_n_checkpoints method in utils.py file.

[vwxyzjn]

Thanks for the PR. What is the meaning of --keep_last_n_checkpoints -1?

[shizhengLi]

Hi, in the open-instruct/open_instruct/finetune.py file, there is a parameter:
keep_last_n_checkpoints: int = field( default=3, metadata={"help": "How many checkpoints to keep in the output directory. -1 for all."}, )

--keep_last_n_checkpoints -1 means keeping all the checkpoints. When I set it, it didn't work, so I just fixed it.