enable mypy + backfill type annotations

[MichelleArk]

No description provided.

[emmyoop]

I've done some mypy cleanup in #21, but not all. This issue is still relevant.

[ChenyuLInx]

As discussed during the meeting we want to see if we can make sure every function modified has type def after modification.

[github-actions]

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[github-actions]

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