changes.
*   `RNNCell` objects now subclass `tf.layers.Layer`. The strictness described
    in the TensorFlow 1.1 release is gone: The first time an RNNCell is used, it
    caches its scope. All future uses of the RNNCell will reuse variables from
    that same scope. This is a breaking change from the behavior of RNNCells in
    TensorFlow versions <= 1.0.1. TensorFlow 1.1 had checks in place to ensure