// `map_fn` will trace upwards along the use-def chain of the ssa value. It
  // starts from the last ssa value (returned by the function), and check its
  // parent op iteratively. If the root ssa value appears in the function's
  // argument list, it will return the index of the corresponding argument,
  // otherwise it will return -1.
  auto map_fn = [](Value value) -> int {
    Value parent = value;

        are applied to ExtensionType values.
    *   The `BatchableExtensionType` API can be used to define extension types
        that support APIs that make use of batching, such as `tf.data.Dataset`
        and `tf.map_fn`.
    *   For more information, see the
        [Extension types guide](https://www.tensorflow.org/guide/extension_type).

## Bug Fixes and Other Changes

*   TF Core:

elements from `input_dataset` in parallel.}]>:$batch_size,
    Arg<TF_Int64Tensor, [{A scalar representing the maximum number of parallel invocations of the `map_fn`
function. Applying the `map_fn` on consecutive input elements in parallel has
the potential to improve input pipeline throughput.}]>:$num_parallel_calls,