*   Set as much partial shape as we can infer statically within the gradient
        impl of the gather op.
    *   Gradient of `tf.while_loop` emits `StatelessWhile` op if `cond` and body
        functions are stateless. This allows multiple gradients while ops to run
        in parallel under distribution strategy.
    *   Speed up `GradientTape` in eager mode by auto-generating list of op