Fix inc quantization bugs

[xiaoyu-work]

Describe your changes

• Unify the logic when bits input is either int or enum.
• Use IntEnum by default since IntEnum is available since python 3.4

Checklist before requesting a review

• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
• Lint and apply fixes to your code by running lintrunner -a
• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.

(Optional) Issue link