[Feature] Add CQL

[BY571]

Description

Implements the Conservative Q-Learning (CQL) objective for Offline Reinforcement Learning (https://arxiv.org/abs/2006.04779)
and adds examples for online and offline training.

Performance tested for Pendulum:

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[vmoens]

Great work! Exciting stuff
A few comments

[vmoens]

Great work! Exciting stuff
A few comments

[vmoens]

Awesoooome!
let's wait for the tests to pass :)

[BY571]

I'm getting an error now regarding the in_keys of the objective, checking about that!

Fixed!