Custom space vectorizers

[Angelo]

As in SB3 (https://stable-baselines3.readthedocs.io/en/master/guide/custom_policy.html#custom-feature-extractor) it would be very helpful if a state shape is very strange and it is not a vector nor an image (for example, it could be a graph, or a dictionary space).

[rodrigodesalvobraz]

Thank you for the suggestion. I've labeled it with "enhancement" so it will appear in a feature request list.