Add MyPy type checking to CI

[m3rlin45]

🚀 Feature Request

let's have typechecking, like in ParlAI: https://github.com/fairinternal/ParlAI-Internal/blob/705258b5799e69ef8942a8fc56928ad779e0980b/.github/workflows/lint.yml

Motivation

It would be good to have some type checks (as suggestions, not a blocking test) to help catch common errors

Pitch

probably can just copy/past from ParlAI:
https://github.com/fairinternal/ParlAI-Internal/blob/705258b5799e69ef8942a8fc56928ad779e0980b/.github/workflows/lint.yml

https://fb.workplace.com/100020258253907/posts/744791912872744/
Some slides about the ParlAI version

[TommyDew42]

Hi! It looks like a very good issue I can contribute. Is it possible I can pick this up?

[stephenroller]

Please Tommy, we would welcome!

[punitkoura]

Looks like this is done, closing