fix: do not leak set_attention_backend into process-wide registry

[Solaris-star]

What does this PR do?

ModelMixin.set_attention_backend() already pins the backend on this model's attention processors. It also called _AttentionBackendRegistry.set_active_backend(), which is process-global.

Any other model whose per-module backend is still None then falls through to that global backend in dispatch_attention_fn. Example: model A sets FA3; model B later runs with attn_mask and unexpectedly hits FA3, which rejects the mask.

This PR stops flipping the global registry from set_attention_backend(). Per-module pins stay. Use attention_backend(...) when a temporary process-wide override is intentional.

Fixes #14249

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Self-review notes

• Root cause is global registry mutation, not missing per-module assignment.
• Change is intentionally small: remove one global write; keep per-module loop.
• No new tests in this PR yet; happy to add a unit test that asserts active backend is unchanged after set_attention_backend if maintainers want it.
• Docs: behavior of attention_backend() context manager is unchanged.

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