[GG clean] fix(dcp): preallocate packed A2A graph buffers

[voipmonitor]

Summary

Reapply the useful part of previous PR #130 after resetting dev/gilded-gnosis back before the Jul 20 merge/revert block.

This keeps the DCP packed A2A graph-buffer lifetime fix, without bringing back the discarded CUTLASS pin or the old MLA .contiguous() workaround.

Original PR

• Previous merged PR: [GG] fix DCP packed A2A buffer lifetime during graph prewarm #130
• Original commit cherry-picked with provenance: ef7cae43b2

Changes

• Preallocate/retain packed A2A send/recv graph buffers when CUDA graph capture is active.
• Add unit coverage for graph prewarm buffer retention and eager non-retention behavior.

Merge Order

This is the first PR in the clean GG reapply stack.

Do not merge until explicitly approved.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 371085e and 3fe930a.

📒 Files selected for processing (2)

• tests/distributed/test_dcp_a2a.py
• vllm/v1/attention/ops/dcp_alltoall.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/gg-dcp-a2a-prewarm-clean-20260720

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[voipmonitor]

Closing as already represented in the current dev/gilded-gnosis base. The packed A2A graph-buffer prewarm behavior and its tests are present there via 639aeff, so this conflicting branch has no unique change left to merge. The follow-on lifecycle work is consolidated in #149.