optimize agent save

[huangshiyu13]

🐛 Bug

agent.save() is not well implemented. The saved file for nlp task is too large.

To Reproduce

from openrl import ...

Relevant log output / Error message

No response

System Info

v0.0.7

Checklist

• I have checked that there is no similar issues in the repo
• I have read the documentation
• I have provided a minimal working example to reproduce the bug
• I have version numbers, operating system and environment, where applicable