[Minor] Expose more APIs for loading and saving

[frankaging]

Description

Exposing more APIs for loading and saving with trainable model components.

Testing Done

Nothing will be effected. Run P0 tests.

Checklist:

• [x] My PR title strictly follows the format: [Your Priority] Your Title
• [x] I have attached the testing log above
• [x] I provide enough comments to my code
• [x] I have changed documentations
• [ ] I have added tests for my changes