Tuturial: DQN and Double DQN with network instance

Summary:
This simple example illustrates how users can use implementations of value based methods, specifically, DQN and Double DQN, with gym environments.

In Pearl, we also allow users to pass network instance to value based methods. This example illustrates how to use this functionality in Pearl.

Reviewed By: rodrigodesalvobraz

Differential Revision: D53947100

fbshipit-source-id: 7b1a437e37cbb746565f0c283f1bb7905d6af137