[Bugfix][Spec Decode] Fix DP metadata for DFlash profiling

[Vaishnavi220506]

Summary

Fixes #56281.

During memory profiling, the DFlash/DSpark dummy draft pass uses num_query_tokens, but it previously passed None for num_tokens_across_dp. With DP > 1, set_forward_context then performs a DP coordination step using the target model's token count, which can leave DPMetadata.make() asserting on a stale count (for example, 4096 20).

This change creates a draft-sized per-rank token-count tensor from the existing DPSyncState and passes it into the dummy draft forward. This keeps the dummy pass aligned with its actual workload and avoids the incorrect DP coordination path. The regression test verifies that target counts are replaced by the draft query count.

Why this is not duplicate work

Issue #56281 is still open, unassigned, and had no competing open pull request when this branch was created. This is a focused regression fix for the interaction between the DFlash dummy-run path and the target parallel configuration changes in #54856 and #55472. It is unrelated to the earlier closed PR #55464.

Validation

• python -m compileall -q vllm/v1/worker/gpu/spec_decode/dflash/speculator.py tests/v1/spec_decode/test_dflash2.py — passed
• python -m ruff check vllm/v1/worker/gpu/spec_decode/dflash/speculator.py tests/v1/spec_decode/test_dflash2.py — passed
• python -m pre_commit run --files vllm/v1/worker/gpu/spec_decode/dflash/speculator.py tests/v1/spec_decode/test_dflash2.py — all applicable hooks passed; the Dockerfile graph hook could not run because this Windows environment has no /bin/bash
• The focused pytest could not collect in this Windows environment because the repository requires the unavailable uvloop package; no CUDA multi-GPU runtime is available here for the DeepSeek V4 reproduction

The DeepSeek V4 DP > 1 + EP + DSpark startup reproduction should be rerun by CI/maintainers on the supported CUDA setup.

AI assistance disclosure

AI assistance was used to inspect the issue and related changes, propose the minimal implementation, and draft the regression test. The submitter is responsible for reviewing and understanding every changed line and for validating the behavior on the supported CUDA configuration.

[claude]

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[Vaishnavi220506]

The initial pre-run-check completed as expected for a first-time contributor: it is blocked by the repository author/label gate (0 merged PRs and no maintainer-issued ready/verified label), not by a code or test failure. Local compile, Ruff, formatting, mypy, and applicable pre-commit hooks pass; the focused pytest is blocked only by missing Windows dependencies (uvloop, then cpuinfo) and no CUDA multi-GPU runtime is available here. The PR is intentionally limited to the DP metadata fix plus its regression test. Maintainers can review and run the supported CUDA validation when appropriate.

[Vaishnavi220506]

Closing this PR after upstream maintainer verification on issue #56281.

The regression was caused by #53694, not #54856, and the correct fix is already present on upstream main (verified on DeepSeek V4 Flash + DSpark with DP > 1). Because the upstream fix makes this branch duplicate work, I’m closing it rather than asking maintainers to review or merge redundant code.