[tf.data] Adding support for `num_parallel_calls` to `tf.data.Dataset.interleave`.

Unlike the `tf.data.contrib.parallel_interleave` whose parallelism is tied to the `cycle_length` argument, the newly introduced `num_parallel_calls` argument of `tf.data.Dataset.interleave` is decoupled from the `cycle_length` argument and identifies the degree of parallelism to use for fetching output elements.

PiperOrigin-RevId: 211886816