[tf.data] Introducing an optimization that parallelizes map transformations.

Stateless MapDatasets can be paralellized by switching to ParallelMapDataset. We set `num_parallel_calls` to 2 for now, but in the future a special value will be used that result in the optimal value to be selected dynamically at runtime.

This patch also exposed a memory leak which was fixed.

PiperOrigin-RevId: 213015223