*   `tf.contrib.estimator.InMemoryEvaluatorHook` and
        tf.estimator.experimental.InMemoryEvaluatorHook`.
    *   `tf.contrib.estimator.make_stop_at_checkpoint_step_hook` with
        `tf.estimator.experimental.make_stop_at_checkpoint_step_hook`.
    *   Expose `tf.distribute.Strategy as the new name for
        tf.contrib.distribute.DistributionStrategy.
    *   Migrate linear optimizer from contrib to core.