[Qwen 3] promote_dtype_for_stats flag to optionally casting RMSNorm to FP32

[JiriesKaileh]

Description

Setting promote_dtype_for_stats=False keeps RMSNorm at BF16.
Setting promote_dtype_for_stats=True casts RMSNorm to FP32.

Tested perf and accuracy differences in this bug

[github-actions]

Description

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[jrplatin]

Also, please include the 1k/4k and MMLU results here (with repro commands) in the Testing seciton, thanks!