Address `mypy` type hinting issues

[Andrew-S-Rosen]

We won't be able to address them all in one PR, but steadily we should try resolving them by running mypy on the codebase and addressing errors as they arise.

[abastola0]

@Andrew-S-Rosen can you please assign me on this issue?

[jayanthkoduri]

@Andrew-S-Rosen I need to contribute to Open Source as part of my Course work. Can you please assign me this.

[Andrew-S-Rosen]

@abastola0, @jayanthkoduri: Feel free to work on a PR. I don't assign issues. This issue is large enough where it won't be possible to solve it completely with one PR.

If you both plan to work on this, I recommend commenting here about which modules you plan to work on so you don't overlap. Like I mentioned, don't try to do them all.