[Deepspeed] skip device mesh creation when deepspeed and sp_size >1

[kashif]

What does this PR do?

Accelerate should skip DeviceMesh creation when sp_backend == "deepspeed" because DeepSpeed handles all SP groups internally through UlyssesSPAttentionHF.register_with_transformers().

Fixes # (issue)

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[SunMarc]

LGTM !