Pegasus model plug-in rc1 (#701)

* added new RealFace training model (codenamed: Pegasus)

* minor re-factoring (doesn't break compatibility)

* fixed tf error for input/output sizes with ratios other than 1:2

* linting

* Minor formatting/grammar ammends

* Make loss a fixed config item. Make learning rate an unfixed item

* Check input and output sizes