support backuping multiple single files

[liyc-ai]

This PR tries to support backuping multiple run files.

Currently, RLA supports backuping code dir or single run file. However, there exists the need to backup multiple single files. To support this, I modify the type of run_file from str to be str or List[str]. Right now, to backup multiple single files, we just need to wrap them in a list, like below:

rla_run_file = ['train.py', 'utils.py', 'draw.ipynb']

Then everything will be OK.

By the way, this PR also tackles a bug when the backup_code_dir in rla_config.yml is None.

[xionghuichen]

nice work~