*   Triangular solve performance improvements.
    *   Unify RNN cell interface between TF and Keras. Add new
        get_initial_state() to Keras and TF RNN cell, which will use to replace
        the existing zero_state() method.
    *   Update initialization of variables in Keras.
    *   Updates to "constrained_optimization" in tensorflow/contrib.
    *   boosted trees: adding pruning mode.