Clean the repo by removing the deprecated modules and merging the files with the same functionalities.

[DavdGao]

Description

• Merge utils/common.py and utils/tools.py into utils/common.py
• Prefix inner functions with "_"
• Remove deprecated PromptEngine module, which is declared to be deprecated in v0.0.6.
• Remove text-to-image agent, which is declared to be deprecated in v0.0.6.
• Improve OpenAI services to avoid unnecessary imports.
• Remove empty files test.py and file_manager.py in the root directory

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