Revert part of https://github.com/yandexdataschool/Practical_RL/pull/448 (#462)
Revert part of https://github.com/yandexdataschool/Practical_RL/pull/448 (#462)

* Revert part of https://github.com/yandexdataschool/Practical_RL/pull/448

* Add a note about running Q-learning using TF on CPU
yandexdataschool/Practical_RL,yandexdataschool/Practical_RL