Add new package keras_contrib

[Franky1]

https://github.com/keras-team/keras-contrib

[rosbo]

We will move soon to TensorFlow 2.0. The recommended way to use Keras will be to use tf.keras.

keras.io as of yesterday (9/17/2019) supports TensorFlow 2.0 as a backend but not all features and the recommended way to use keras is through tf.keras:

This release brings the API in sync with the tf.keras API as of TensorFlow 2.0. However note that it does not support most TensorFlow 2.0 features, in particular eager execution. If you need these features, use tf.keras.

-- https://github.com/keras-team/keras/releases/tag/2.3.0

For this reason, we won't add keras-contrib at this points since it relies on the keras.io package.

Thank you