Improve type hints for backends

[waketzheng]

Description

Improve type hints to reduce type:ignore

Motivation and Context

Only make style and improve type hint without changing any code logic

How Has This Been Tested?

make test

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added the changelog accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.