*   symbolic tensors outside the scope of the model are used in custom loss
        functions. The flag can be disabled for these cases and ideally the
        usage pattern will need to be fixed.
    *   Mark Keras `set_session` as `compat.v1` only.
    *   `tf.keras.estimator.model_to_estimator` now supports exporting to
        `tf.train.Checkpoint format`, which allows the saved checkpoints to be
        compatible with `model.load_weights`.