Refactored ContinuousCritic for SAC/TD3

[araffin]

Description

Preparing #3, it merges SAC and TD3 critics and it will be used by DDPG soon.
I added a parameter n_critics, even though SAC/TD3 only support n_critics=2 for now. (It will be mainly useful for DDPG)

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

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

Checklist:

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (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.
• I have checked the codestyle using make lint
• I have ensured make pytest and make type both pass.

[Miffyli]

Left couple of comments, just doc stuff. Other than that it looks good to me 👍

[araffin]

@Miffyli sorry, I did my edits last week but did not pushed them...

[Miffyli]

Looks good to me :)

sorry, I did my edits last week but did not pushed them...

No biggie, happens to all of us :D