[II] Gather paired DCP projections behind one barrier

[voipmonitor]

Status

Qualified.

Behavior

dcp_b12x_all_gather_pair gathers two contiguous rank-local projection rows through one B12X IPC synchronization epoch. FP16, BF16, FP32, and FP8 E4M3 rows are accepted when each row occupies a multiple of 16 bytes.

Unsupported shapes, dtypes, capacities, alignments, world sizes, or B12X availability use two exact process-group gathers in the original tensor order.

Technical reason

Decode pipelines often gather a latent projection and a router projection together. Separate collectives pay two system-scope barrier epochs even though both tensors have the same batch and process group. B12X already exposes a paired gather that shares the barrier.

Compatibility

The public helper has an exact fallback and does not change existing single-tensor gather behavior. The change depends on the DCP16 dispatch guard in #338. B12X DcpAllToAllPool.all_gather_pair already provides the transport primitive.

Validation

• Ruff formatting and checks pass.
• git diff --check passes.
• Focused tests cover TP16/DCP16 paired dispatch, mixed BF16/FP32 rows, 16-byte alignment rejection, exact fallback ordering, and DCP16 warmup.

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[voipmonitor]

The behavior implemented by this pull request is preserved in vLLM #384 as commit 810bbc807dc1. The stable patch ID differs because the paired gather returns the caller-owned output created by the consolidated gather path. The consolidated range-diff retains the pull request contract. Review and merge #384; this pull request is closed to avoid duplicate review.