[None][perf] Remove redundant allreduce

[mikeiovine]

Description

Skip a redundant allreduce.

1. When attention DP is off and TP > 1, the DSV4 attention op reduces its output in o_b_proj.
2. This reduced output would then go through the fused_hc
3. We would then do another fused all reduce sum + RMSNorm. This does not affect correctness because RMSNorm with positive epsilon is roughly scale invariant.
4. The extra allreduce is redundant here.

This PR changes the flow. The RMS norm is folded into fused_hc, and the extra allreduce is eliminated.

Test Coverage

Existing tests.

TPS/User improves ~3% at batch size 1 (DSV4 Pro, 8xB300) in my local tests.

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