Fix vanilla DSpark checkpoint exports for vLLM - #2902

Actionable comments posted: 1

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Review comments at @modelopt/torch/export/plugins/hf_spec_export.py:
- Around line 589-600: Update DFlashExporter’s hf_quant_config generation before
writing config.json and hf_quant_config.json to rename relevant exclude_modules
and quantized_layers references from dflash_module.markov_w1,
dflash_module.markov_w2, and dflash_module.confidence_proj to the vanilla DSpark
paths markov_head.markov_w1, markov_head.markov_w2, and confidence_head.proj.
Apply this mapping only for the vanilla markov_head_type and preserve unrelated
quantization references.

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LGTM: the export remap and backward-compatible loading look correct; the justified existing-test updates require human sign-off.

Needs action:

• Confirm the updated expectations in tests/unit/torch/speculative/plugins/test_hf_dspark.py: vanilla exports legitimately change architecture and head names, while legacy loading and gated/RNN coverage remain intact.

No action needed:

• Tests cover confidence enabled/disabled, uppercase vanilla normalization, duplicate-key precedence, warm-start round trips, and remapped shape rejection.
• Tests were inspected, not executed in this review; the PR reports focused CPU tests and stock vLLM 0.32 GPU smoke validation.

Reviewed the exporter and compat changes. The fix is the right shape and the core claim holds up — but I'd want one question answered before it lands, and the branch needs a rebase regardless.

Verified

• Qwen3DSparkModel is an accurate name, not a guess. modeling_dflash.py:39-53 builds the draft from transformers.models.qwen3 (MLP, RMSNorm, RotaryEmbedding) "matching z-lab's reference checkpoint format", so the backbone really is Qwen3 whatever the teacher is. The PR's "teacher can still be GPT-OSS" framing is correct.
• Sourcing markov_head_type from self.model.dflash_module is an improvement, not just a move. modeling_dspark.py:91 normalises and lowercases it and :98 validates it against (vanilla, gated, rnn), whereas the old getattr(draft_config, "markov_head_type", "vanilla") would silently treat "Vanilla" as non-vanilla and emit the native layout under a config claiming otherwise. Worth keeping even independently of this fix.
• Gating the rename and the architectures switch on the same normalised value means config and weights cannot disagree about which layout was written.

The question I'd want answered

sample_from_anchor: True and confidence_head_with_markov appear nowhere else in this repository — not in a config, a recipe, a test, or another exporter. They are pure vLLM-side contract, hardcoded here with nothing in-tree to validate them against.

enable_confidence_head and confidence_head_with_markov are both set from use_confidence_head, so today they cannot disagree; if they are genuinely one flag on the runtime side, that is fine but worth a word. sample_from_anchor: True is the one that concerns me: it is unconditional, so if ModelOpt ever trains a DSpark draft whose anchor behaviour differs, the exported config asserts something about the checkpoint that nothing checks. A comment naming the vLLM version and the loader these keys come from would make the contract auditable — the same way the DFlashGroupedConv import comment names its dependency.

Smaller notes

Two sources of truth in the emitted config. markov_rank, mask_token_id, target_layer_ids and the confidence flag are now written both nested inside dflash_config and at top level. Required by the runtime, I assume — but a later edit to one is not forced to update the other, and nothing would catch the drift. If the top-level block is purely vLLM's view, deriving it from config["dflash_config"] at the end (as the mask/layer-ids lines already do) rather than re-reading draft_config for markov_rank would at least keep one of them canonical.

Naming asymmetry, probably deliberate. The other exporters in this file use family names — DFlashDraftModel (:370), LiLiCorrDraftModel (:548), DFlash2DraftModel (:630). This introduces the first vendor-prefixed one. That is presumably forced by how vLLM selects its loader, which is exactly the bug being fixed, so I would not change it — but it is worth one line in the docstring saying the name is dictated by the runtime's dispatch rather than by our architecture naming, so nobody "fixes" it later for consistency.

Before merge

mergeable: CONFLICTING / mergeStateStatus: DIRTY against main — needs a rebase. Flagging early because I also have #2313 open against hf_spec_export.py; it touches the module-level helpers and SpeculativeDecodingExporter rather than DSparkExporter, so the two should resolve mechanically, but whichever lands second will need to look.

I did not run the tests, and I cannot check the vLLM 0.32 behaviour the PR validates on B200 — taking the GPU smoke results at face value.