[Feature Request] Allow Gymnasium Composite Spaces

[flowerthrower]

🚀 Feature

Allow gymnasium composite spaces like gymnasium.spaces.Sequence or gymnasium.spaces.Graph when providing a custom feature extractor (which supports those).

Motivation

Users that create a custom feature extractor to map observations of variable size (e.g. through graph or recurrent NNs) to fixed size output vectors can still not use policies like PPO without having to modify SB3 checks and constraints that prevent the user from using a variable size environment.

Pitch

This does not require models to support those spaces but only to allow the user to use them with custom provided feature extractors.

Alternatives

No response

Additional context

No response

Checklist

• I have checked that there is no similar issue in the repo
• If I'm requesting a new feature, I have proposed alternatives

[araffin]

Duplicate of #1688 #1723 and #1280

Are you willing to contribute such feature?

[flowerthrower]

I am afraid my current understanding of the package is too shallow for contributing right now.
Since this is one of multiple duplicates I hope the original request can be implemented in the future.