[Proposal] Allow frame stack for stacks of size 1

[albertwilcox]

Proposal

Hi,

I propose that the framework is updated to allow frame stacks of size 1. This could be helpful for pipelines which might use frame stacks in some settings and omit them in others. By allowing frame stacks of size 1 the frame stack wrapper can be left in the code and automatically add the extra dimension for frame stacking.

To implement this, simply remove these lines:

Gymnasium/gymnasium/wrappers/stateful_observation.py

Lines 377 to 380 in a4f1a93

	if not 1 < stack_size:
	raise ValueError(
	f"The stack_size needs to be greater than one, actual value: {stack_size}"
	)

Previous versions of the gym wrapper supported this: https://github.com/openai/gym/blob/master/gym/wrappers/frame_stack.py

Motivation

As described above, this will enable cleaner code for several applications without causing any issues to existing ones. Also, this is consistent with previous versions of the gym frame stack wrapper.

Pitch

Just remove these lines or maybe assert stack-size > 0 instead

https://github.com/Farama-Foundation/Gymnasium/blob/main/gymnasium/wrappers/stateful_observation.py#L377-L380

Alternatives

No response

Additional context

No response

Checklist

• I have checked that there is no similar issue in the repo

[pseudo-rnd-thoughts]

Hey, thanks for the suggestion, I originally included the >1 as I thought you wouldn't need ==1 but your motivation is good counter-argument.
If the tests work then I see no reason why we can't relax the assumption.
Could you make a PR for this?