Support n_groups>1 for mamba2

[tdoublep]

What does this PR do?

This fixes an issue for models like mistralai/Mamba-Codestral-7B-v0.1 that use mamba2 architecture with n_groups>1. It is equivalent to the fix for Zamba from #35943.

This issue prevents us comparing against transformers as baseline in vLLM CI for this model.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@pglorio @vasqu @ArthurZucker @hmellor

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: bamba, falcon_h1, granitemoehybrid, mamba2

[vasqu]

Can we also let the zamba2 model inherit this from mamba2 then? Let's not introduce alternatives here

And could you add a fast test to mamba2 that might catch this?

[vasqu]

Suggested change

	def __init__(self, hidden_size, group_size, eps=1e-6):
	super().__init__(hidden_size, group_size, eps)
	pass

Weird that this even used the init here, we shouldn't need to anything on the modular side

[vasqu]

Same for the other modular codes