*   `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.
    *   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.