*   Promoting `tf.data.experimental.unique` API to `tf.data.Dataset.unique`
        and deprecating the experimental endpoint.
    *   Added `stop_on_empty_dataset` parameter to `sample_from_datasets` and
        `choose_from_datasets`. Setting `stop_on_empty_dataset=True` will stop
        sampling if it encounters an empty dataset. This preserves the sampling
        ratio throughout training. The prior behavior was to continue sampling,