_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,
        then bazel will build TensorFlow API version 2.0. Note that TensorFlow
        2.0 is under active development and has no guarantees at this point.