*   For `tf.Tensor` & `tf.Variable`, `.experimental_ref()` is no longer
        experimental and is available as simply `.ref()`.
    *   `pfor/vectorized_map`: Added support for vectorizing 56 more ops.
        Vectorizing `tf.cond` is also supported now.
    *   Set as much partial shape as we can infer statically within the gradient
        impl of the gather op.