[RLlib] Fix small bug in 'InfiniteLookBackBuffer.get_state/from_state'.

[simonsays1980]

Why are these changes needed?

When get_state is called it tries to turn the space_struct into a dictionary by using gym_space_to_dict. This errors out for composite spaces because the outer space is not a space but a container (i.e. the sample fo that space). This PR fixes this error, but focusing on the space attribute which isa true gymnasium.spaces.Space in its outer space and rebuild the space_struct in from_state when existent.

Related issue number

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[sven1977]

LGTM. Thanks for the fix @simonsays1980 .