Don't hide utils from the API docs generator

[yashk2810]

Utils contains types/ and since the API generator now supports type annotation with links, it will be good to link off to the type pages.

[MarkDaoust]

Would it make sense to import that utils.types in the root init.py so you don't document the rest of utils too?

Or switch to that explicit package thing and add import tensorflow_addons.utils.types to utils?

Can you post a diff of the output TOC either way?

[gabrieldemarmiesse]

Maybe you'd need to bump the version for the CI to pass? See https://github.com/tensorflow/addons/blob/master/tools/install_deps/doc_requirements.txt

[facaiy]

I think utils module is hidden intentionally, because we don't want to guarantee its backward compatibility.

[WindQAQ]

Should be done in #2162