Adding types information on the API surface.

Adding types on the public API surface would allow us to do some runtime type checking later on and would allow user's IDE to have more info for static analysis.

The functions/signatures to type are the ones listed here https://github.com/keras-team/autokeras/blob/master/autokeras/__init__.py

For the context, see #856 where I add some type information on a ImageClassifier.

This issue can be considered easy to solve (good first issue), it's just long to do because the public API surface is big.

I'll do it but some help is welcome :) make sure to make small pull requests.