[guide](https://www.tensorflow.org/guide/concrete_function) for more
        details on `concrete_ function`.
    *   Update `tf.function`'s `experimental_relax_shapes` to handle composite
        tensors appropriately.
    *   Optimize `tf.function` invocation, by removing redundant list converter.
    *   `tf.function` will retrace when called with a different variable instead
        of simply using the `dtype` & `shape`.