qwen4exp: support draft-head-only GGUFs (unsloth layout) + fix draft-load regression

[TheArchitectit]

Overview

This patch lets --spec-type draft-mtp work with the draft-head-only GGUFs
that unsloth ships for Qwen3.8-Flash-Next, and fixes a one-line bug where the
draft loader used the target model's path instead of the -md path. It is a
companion to #27836 (qwen4exp architecture + NextN/MTP speculative decoding)
and depends on that PR landing.

Both problems showed up while trying to run speculative decoding on a
147 GB MoE model on a pure-CPU box: the shipped draft pack does not fit
what #27836's loader demands, and the draft-load bug made it impossible to
use a separate draft file at all.

Fix 1: draft-head-only GGUFs

#27836's qwen4exp loader requires hc_head_norm/hc_head_down/hc_head_up
and the PLE block unconditionally. The unsloth GGUFs ship draft-head-only
files that lack these tensors — the target carries output_hc_* in its
shards; the draft pack has only nextn.shared_head_norm + the block's own
ffn projections.

The patch adds an mtp_only probe (n_layer_nextn > 0 && blk.0.hc_attn_norm absent) and, when it fires:

• trunk hc_head_* tensors marked TENSOR_NOT_REQUIRED when mtp_only
• PLE block guarded behind !mtp_only
• trunk tensor loop skipped when mtp_only (layers 0..n_layer-1 absent)
• MTP graph falls back to shared_head_norm + hc_ffn_down/hc_ffn_up
  when the head's own mixer is absent (mirrors the fallback path in the
  reference implementation)
• nextn.hc_head_* marked TENSOR_NOT_REQUIRED; new nextn.shared_head_norm
  tensor loaded when present

Fix 2: draft-load regression

common/speculative.cpp:2585 loaded params.model.path (the target
model path) instead of model_path (the draft model path) — the variable
holding the user-specified -md path. This caused the 147 GB target model
to be loaded again as the draft, consuming all RAM and failing. One-line fix.

Additional information

Keeping this PR in draft until #28389 (my other open PR, cuda: fix CUB argsort
corruption) is reviewed, since new contributors are limited to one open PR.
It is independent of #28389 code-wise and waits on #27836 regardless.

Testing

Measured on a 48-core Haswell Xeon (E5-2660 v3) running Qwen3.8-Flash-Next
(UD-Q4_K_XL target, 4-shard GGUF, ~147 GB mlock), with the native MTP head
requantized to Q8_0 as the draft model.

# upstream binary + PR #27836 + this patch
--spec-type draft-mtp --spec-draft-n-max 4 -md mtp-qwen4exp-Q8_0draft.gguf

# 8-prompt diverse suite (ab_8prompt.py)
ON  (draft-mtp Q8_0): mean 4.28 t/s, acceptance 0.51–0.75, mean_len 3.0–4.0
OFF (stock twin):     mean 3.33 t/s
Δ: +29% mean, 2.0× peak (best prompt 6.62 t/s vs 3.33)

Comparing the per-stream combiner from #27836 against a mean-pooled variant
(same head, same flags): acceptance was 0.51–0.75 vs 0.18–0.20. That lines
up with the note in #27836 that the combiner has to run per hc stream.

Drafter quant ladder (same upstream binary, dn=4)

Each rung is a requantization of the native BF16 MTP head pack (bit-identical
lineage); I arm-tested every one with a journal-verified load.

Quant	Size	Suite t/s	Mean acc_len	vs stock
Q8_0	3.9 GB	4.28	2.25	+29%
Q4_K_M	2.8 GB	3.97	2.34	+19%
Q5_0	2.8 GB	3.95	2.40	+19%
Q3_K_M	2.2 GB	3.44	2.15	+3%
Q2_K	1.8 GB	3.44	2.07	+3%

Acceptance is only the limiting factor below ~5 bits; every rung beats stock
(3.33 t/s). Q8_0 is the practical pick (identical speed to F16 at half the
RAM; F16 ties at 4.29 t/s with highest acceptance 2.51 but costs 7.3 GB).

Draft-n-max sweep (Q8_0)

dn	Suite t/s	Mean acc_len	vs stock
2	3.72	1.88	+12%
4	4.28	2.25	+29%
6	3.52	2.47	+6%
8	3.38	2.60	+2%

dn=4 is the sweet spot — past that, the extra verify work costs more than
the acceptance it buys back.

Files changed

This branch carries three commits from #27836, so the GitHub diff against master shows 10 files. The changes authored here are two:

• src/models/qwen4exp.cpp — mtp_only detection, optional tensors, graph fallback (+25 lines)
• common/speculative.cpp — draft-load path fix (1 line)

The remaining files in the diff (conversion/qwen4exp.py, gguf-py/gguf/constants.py, gguf-py/gguf/tensor_mapping.py, src/llama-arch.{h,cpp}, src/llama-model.{h,cpp}, src/models/models.h) come from #27836 and are shown for merge context only.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES — an AI assistant (Claude Code) helped polish the wording of this description and worked alongside me on the validation runs; the design, measurements, and every number above are my own, and I take responsibility for all submitted changes.

[ggml-gh-bot]

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[TheArchitectit]

Status/plan note: per the automated review on #28389 (new contributors are limited to 1 open PR at a time), I am keeping this PR in draft until #28389 (cuda: fix CUB argsort corruption) is reviewed/approved. It is independent of #28389 code-wise; this one is a companion to #27836 and depends on it landing regardless. I will also update the description to follow the PR template. No action needed from maintainers on this PR in the meantime.

[pwilkin]

/bot review

[ggml-gh-bot]

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