Using tf.contrib.layers.optimize_loss in model_fns (estimators)

Guys, there is a neat function in tensorflow v1 which takes care of a lot of biolerplates in estimators: https://www.tensorflow.org/versions/r1.12/api_docs/python/tf/contrib/layers/optimize_loss

If you guys, don't mind, I will add this to our estimators. It might break backward compatibility in terms of not being able to resume trainings from older checkpoints.