REFAC: Rename lr to learning rate

Rename lr with learning_rate as this is deprecated.

 /usr/lib64/python3.6/site-packages/tensorflow/python/keras/optimizer_v2/optimizer_v2.py:375: UserWarning: The `lr` argument is deprecated, use `learning_rate` instead.
    "The `lr` argument is deprecated, use `learning_rate` instead.")