refactoring into smaller piecies

[deep-diver]

decouple files under model directory

• common.py to define common variables like key names (i.e. IMAGE_KEY)
• utils.py to define common utility function like log and transformed_key
• hyperparams.py to define a set of default hyperparameters
• signatures.py to define the signature for the serving_default.

[deep-diver]

Nice work! Could you add a README or brief descriptions at the top of each of the individual scripts to denote what they're responsible for doing?

Absolutely!

[deep-diver]

@sayakpaul

addressed your comment :)