Add Dict Obs support for DQN

[suijth]

Description

Add MultiInputPolicy for DQN.

Tried to make minimal changes to support this. Did separate out extractors like sb3's MultiInputPolicy

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.

Note: we are using a maximum length of 127 characters per line

[araffin]

Hello,
thanks for the PR.
When you check items in the checklist (especially all the `make' commands), it means you have executed them, not that you will let the CI do the job.

Please avoid any type: ignore unless necessary too.

[suijth]

Hello, thanks for the PR. When you check items in the checklist (especially all the `make' commands), it means you have executed them, not that you will let the CI do the job.

My bad. I was using ubuntu's mypy which was not catching these errors not sure why. I'll probably create a new branch without these commits, it'll be cleaner.

Please avoid any type: ignore unless necessary too.

Noted. Removed them at most places.