fix mypy warning and enforce it in scripts

[Lee-W]

After running mypy locally, I found there are type errors and even some logic errors. Since we already have type hint in our project, it seems to be better if we can fix them now.

Plan

• [x] Step 1. Fix current type error and logic errors
• [x] Step 2. Enforce mypy type check on GitHub Action
• [ ] Step 3. Introduce a stricter mypy config and remove vague type like Any

[Lee-W]

Step 1 and 2 is done by #156

[woile]

mypy is strict enough I think, please reopen if otherwise

[Lee-W]

Sounds good 👍