unify preprocess layer in model zoo

[QiJune]

I find that there is some keras preprocessing layer in census_wide_deep_model. Could we move these preprocessing layer into elasticdl preprocessing directory? Or if the preprocessing layer has been implemented already, we could import it from elasticdl preprocessing library.

[workingloong]

Yes, we can import from elasticdl_preprocessing in census_wide_deep_model.