all symbol changes can be found
    [here](https://docs.google.com/spreadsheets/d/1FLFJLzg7WNP6JHODX5q8BDgptKafq_slHpnHVbJIteQ/edit#gid=0).
    *   API clean-up, included removing `tf.app`, `tf.flags`, and `tf.logging`
        in favor of [absl-py](https://github.com/abseil/abseil-py).
*   No more global variables with helper methods like
    `tf.global_variables_initializer` and `tf.get_global_step`.