Add CNN support for DQN

[araffin]

Description

Add CnnPolicy to DQN.

Performance test (on-going, looking good): https://wandb.ai/openrlbenchmark/sbx?nw=nwuseraraffin

Note: it is 10x faster than SB3 DQN =) will be useful for DLR-RM/stable-baselines3#1622

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 reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

Note: You can run most of the checks using make commit-checks.

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[jan1854]

Hi @araffin, I am a bit busy with some projects at the moment, but I try to have a look at this by Thursday.

[jan1854]

Looks good to me 👍