Decouple model definition in model zoo from input features and transform logic.

[brightcoder01]

Make sure the model definition from model zoo can be compatible with various feature inputs and transform logic.

Make the model definition in model zoo reusable in the following scenarios:

1. Different input features.
2. Different transform logic defined in various way.
   feature column api
   keras preprocess layer or customized keras layer
   Internal library

How to abstract the transform component in order to take all three ways above into consideration?
Wrapping the transform process (include embedding) into a keras model is a feasible option.