Add SimplifiedLayerNormToRMSNorm surgery

[unnim-qti]

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[xiaoyu-work]

@unnim-qti can you please update this PR? We are going to release new Olive version this Friday and this PR will be included

[justinchuby]

Why not make this a rewrite rule, out of curiosity?

[unnim-qti]

Why not make this a rewrite rule, out of curiosity?

I considered a rewrite rule, but this transform isn’t purely structural.

• It has opset‑dependent ReduceMean semantics, handles two operator variants and requires conditional rewiring of an optional second output with ValueInfo preservation, which is cumbersome to express in a pattern rewrite.
• A ProtoSurgeon was therefore a cleaner and safer fit.

[justinchuby]

Handling value infos and optional output handling should be straightforward with the onnxscript rewriter. I wonder if you saw any blockers?

[unnim-qti]

I didn’t see any hard blockers in the rewriter, but I followed the existing pattern SimplifiedLayerNormToL2Norm, which use ProtoSurgeon to handle variant‑specific wiring, and optional outputs explicitly. Given that precedent and the comparable complexity here, I kept this transform in the ProtoSurgeon style for the consistency.