*   Add model explainability to Boosted Trees.
    *   Support negative positions for tf.substr.
    *   There was previously a bug in the bijector_impl where the
        _reduce_jacobian_det_over_event does not handle scalar ILDJ
        implementations properly.
    *   In tf eager execution, allow re-entering a GradientTape context.
    *   Add tf_api_version flag. If --define=tf_api_version=2 flag is passed in,