Allow user to initialize agent without calling agentscope.init

[DavdGao]

Description

As the title said, avoid to raise error in logger when not initialized by agentscope.init

Checklist

Please check the following items before code is ready to be reviewed.

• [x] Code has passed all tests
• [x] Docstrings have been added/updated in Google Style
• [x] Documentation has been updated
• [x] Code is ready for review