`tf.data.Dataset.enumerate`.
    *   Promoting `tf.data.experimental.unbatch` to core as
        `tf.data.Dataset.unbatch`.
    *   Adds option for introducing slack in the pipeline to reduce CPU
        contention, via `tf.data.Options().experimental_slack = True`
    *   Added experimental support for parallel batching to `batch()` and
        `padded_batch()`. This functionality can be enabled through
        `tf.data.Options()`.