No more globals. No more `tf.Variable`

The notebook on [low level TF API](https://github.com/ageron/tf2_course/blob/master/02_low_level_tensorflow_api.ipynb) needs major revision as TF 2.0 will at the very least discourages the use of globals through `tf.Variable` and very likely have already entirely ditched it in [RFC: Variables in TensorFlow 2.0](https://github.com/tensorflow/community/pull/11).

Note that the official style guide, [Effective TensorFlow 2.0](https://medium.com/tensorflow/effective-tensorflow-2-0-best-practices-and-whats-changed-a0ca48767aff), also explicitly stated: "No more globals!"