Remove use of internal tensorflow API

[tensorflow]

Remove use of internal tensorflow API

[AndreasMadsen]

Yeah, I think you can be sure that developers will look to addons as an example on how to develop add-ons. So I think it is important to set a good example.

[seanpmorgan]

Thanks Andreas! This is a good call out. We initially discussed this in the below thread, and the reason I had preferred the internal API was that it made many things easier to do in an IDE (Verify subclasses match their parent methods, etc.). The TF export is not understood... and is to me a hindrance while developing with TF.

That said... the way we're doing it is incorrect and unstable so we should use the correct API endpoints as this issue requests. It looks like in the future TF may be re-organized and we can have the best of both worlds with e.g. import tensorflow_keras.keras.optimizers

https://groups.google.com/a/tensorflow.org/forum/#!topic/addons/-QEXKmznCdA