[feat] Add gemma RMS AR fusion (#3322)

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## 📌 Description

Verified from framework side. See perf at
https://github.com/vllm-project/vllm/pull/42646

Add weight bias to RMS norm AR fusion to support gemma and qwen3.5 RMS

## 🔍 Related Issues

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## Summary by CodeRabbit

* **New Features**
* Added a GPU benchmark to compare fused vs unfused AllReduce+RMSNorm
performance.
* Introduced an optional weight_bias parameter to AllReduce-fusion APIs
to support alternate RMSNorm scaling.

* **Tests**
* Added a distributed correctness test for Gemma/Qwen3.5-style RMSNorm
AllReduce fusion.
* Extended AllReduce fusion tests to validate weight_bias variants
(e.g., 0.0 and 1.0).

* **Documentation**
* Updated reference traces and docstrings to describe weight_bias
behavior.

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Signed-off-by: jiahanc <173873397+jiahanc@users.noreply.github.com>