The old Keras optimizer will never be deleted, but will not see any
    new feature additions.
    New optimizers (e.g., `Adafactor`) will
    only be implemented based on `tf.keras.optimizers.Optimizer`, the new
    base class.

## Major Features and Improvements

*   `tf.lite`:

    *   New operations supported:
          * tf.unsortedsegmentmin op is supported.