text: Title: Finish the TODO list of classes/functions to type.
  Body: We have a [big todo list of classes/functions to type](https://github.com/keras-team/autokeras/blob/dda2d9de0c602a1f47b45bb26783eacfd9572815/tests/autokeras/typed_api_test.py#L11) (add type hints, see [PEP484](https://www.python.org/dev/peps/pep-0484/) 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: https://github.com/tensorflow/addons/issues/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!
  
   