Add python type checking to all classes

[arostamianfar]

We have adopted PEP-484 for our type checking, but many classes are still missing them. We should add them to all existing classes.

[allieychen]

In Q2, the type checking will be added to public functions and methods. If there were types listed in Args for private methods, they will also be converted as type hints following PEP-484. The unit test scripts will be skipped for simplicity.