Finish the TODO list of classes/functions to type.

[gabrieldemarmiesse]

We have a big todo list of classes/functions to type (add type hints, see PEP484 for more infos about type hints) and the docs: https://docs.python.org/3/library/typing.html

Functions of the list should be typed and for classes, only the contructor (__init__()) should be typed. No need to specify the return type of a __init__() because it's always None.

When you add types for a function/class, you can remove it from the TODO list in the same pull request.

Please make multiple small pull requests. Not a single big one :)

If you don't really understand what is asked:
We did the same procedure in tensorflow/addons here: tensorflow/addons#989 See this issue and look into the pull requests made.

You can always ask if you're wondering what to do. It's fairly easy and a good way to start contributing to autokeras!