Declare Muse Glimmer draft causality instead of widening the DFlash default

[zixi-qi]

Fixes the DFlash causality test failure on build 83669 without changing the causality default that every other DFlash/DSpark drafter relies on.

Problem

Muse-Glimmer-30B-assistant has five sliding_attention layers and declares no causality, so it resolves causal under the layer-type default. This branch handles that by widening _dflash_layer_causal to treat any uniform layer_types as non-causal.

That changes the default for every DFlash and DSpark drafter — gemma4_dspark.py and v1/worker/gpu/spec_decode/dspark/utils.py call the same predicate — and it breaks a pre-existing test. It is the only failure in both spec-decode jobs on build 83669:

job	result
amd-v1-spec-decode-mi300-1	1 failed, 225 passed, 1 skipped, 5 deselected
v1-spec-decode (H200)	1 failed, 216 passed, 15 skipped, 7 deselected

FAILED v1/spec_decode/test_dflash_causality.py::test_dflash_has_any_non_causal[config3-False]
  - AssertionError: assert True is False

Both vendors fail identically because the predicate is pure Python over a config object.

Change

• Revert the _dflash_layer_causal change; the function is now byte-identical to main.
• Default dflash_config = {"causal": False} on MuseGlimmerAssistantConfig, which already exists to supply the vocab_size and use_sliding_window the checkpoint does not carry. A checkpoint shipping its own dflash_config still wins.
• tests/v1/spec_decode/test_dflash_causality.py is left untouched.

Why non-causal is the right value for this head

• transformers' modeling_muse_glimmer_assistant.py sets is_causal = False and builds create_bidirectional_mask / create_bidirectional_sliding_window_mask for both layer types.
• SGLang declares MuseGlimmerAssistantConfig.is_causal = False.
• SGLang's Muse Glimmer PR ([Feature] Add Muse Glimmer model support sgl-project/sglang#34262) widened the default the same way first, then reverted it in Fix DFlash sliding attention causality defaults sgl-project/sglang#34524 after gemma-4-31B-it-DFlash accept length regressed 5.62 → 5.27 and failed its 5.4 threshold.

Checkpoints needing non-default causality already declare it and are unaffected either way: poolside/Laguna-S-2.1-DFlash and poolside/Laguna-XS-2.1-DFlash (all-sliding, causal=True), and nvidia/NVIDIA-Nemotron-3.5-Lightning-30B-A3B-NVFP4-DSpark.

Test plan and results

Unit, with the test file unmodified from main:

$ .venv/bin/python -m pytest -m 'not slow_test' v1/spec_decode/test_dflash_causality.py
10 passed

Behaviour matrix over every checkpoint shape in the wild (all pass): Muse Glimmer → non-causal; all-sliding undeclared → causal (unchanged from main); poolside all-sliding + causal=True → causal; gemma mixed 4/5 → per-layer; all-full → non-causal; layer_types=None → non-causal; checkpoint-declared causal overrides the new default.

Model evaluation

MT-Bench acceptance, meta-models/Muse-Glimmer-30B + -assistant, num_speculative_tokens=16, 80 prompts, temperature 1.0, TP=1 on one GB200. The two arms differ only in the draft's dflash_config.causal:

	non-causal (this PR)	causal	delta
Acceptance rate	13.67 %	11.13 %	+2.54
Acceptance length	3.19	2.78	+14.7 %

Per-position acceptance is identical at position 0 (69.34 vs 69.49) and diverges monotonically after it — pos 4: 12.57 vs 9.02, pos 8: 5.03 vs 1.89, pos 15: 2.13 vs 0.05. Position 0 has no earlier block slots to attend to, so masking cannot affect it; every later slot does. That is the signature expected when intra-block causality is the only variable.

Caveat: one run per arm at temperature 1.0, so the individual figures carry sampling noise. The direction and the per-position structure are well outside it.

Notes

Not a duplicate: this targets the tiezhen/new-model-support branch behind vllm-project#51655 and only adjusts how that branch reaches non-causal drafting for Muse Glimmer.

AI assistance (Claude Code) was used to produce this change and the measurements above.

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[TheEpicDolphin]

LGTM!

One thing to point out is that with this change, we now go into this branch and set layer_ids = [] (because the dflash config has no target_layer_ids): https://github.com/vllm-project/vllm/blob/f80b66f548d855a104c9b2a0527e6c0b1a31750c/vllm/v1/worker/gpu/spec_decode/eagle/eagle3_utils.py#L44-L46. But everything still works because [] is treated as falsey. We probably want to change that condition in the future so that it's safer, but this shouldn't block the PR!