register mixer2_gated_rms_norm ir kernel

[wxsIcey]

Purpose

Register mixer2_rms_norm_gated as a vllm IR op and rewrite Mixer2RMSNormGated.forward_native to dispatch correctly across all tensor-parallel configurations.

The implementation handles four cases:

Case	Condition	Issue	Solution
1	n_groups=1, tp_size>1	Variance must be computed across all ranks (one global norm group, each rank holds only a slice)	AllReduce local sum-of-squares → compute global variance
2	n_groups=1, tp_size=1	No TP, local data is complete	Use IR op directly
3	n_groups>1, n_groups % tp_size != 0	Group boundaries straddle rank boundaries (a rank may hold half a group), local norm is incorrect	AllGather full tensor → normalize locally → slice back to local rank
4	n_groups>1, n_groups % tp_size == 0	Each rank holds an integer number of complete groups, variance can be computed independently	Use IR op directly

Cases 2 and 4 require no collective communication and are handled by the IR op. Cases 1 and 3 require cross-rank communication that cannot be fused into a single kernel, so they are handled with explicit AllReduce / AllGather before calling into local computation.

Because forward_native now covers all cases (including the optimized IR op paths for cases 2 and 4), forward_cuda is fully redundant and can be removed.

Test Plan

Test Result

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