`tf.keras.optimizer.Optimizer.apply_gradients`. This allows custom
        gradient aggregation and processing aggregated gradients in custom
        training loop.
    *   Allow `pathlib.Path` paths for loading models via Keras API.
*   `tf.function`/AutoGraph:
    *   AutoGraph is now available in `ReplicaContext.merge_call`,
        `Strategy.extended.update` and `Strategy.extended.update_non_slot`.