[Bugfix] Tolerate an unset group_size in the Marlin MoE support probe

[afierka-intel]

Summary

check_moe_marlin_supports_config compared group_size against integers:

group_size <= 0
intermediate_size_per_partition % max(64, group_size)

compressed-tensors leaves group_size unset for per-channel strategies, so it arrives as None and both raise:

TypeError: '<=' not supported between instances of 'NoneType' and 'int'

The raise happens inside _backend_incompatibility_reason, whose contract is to return a reason string for an unsupported config. So instead of falling back to another backend, backend selection aborts and the user gets a traceback. In practice this hits compressed-tensors int4 MoE checkpoints with strategy: "channel".

Fix

Normalise None to the -1 this function already accepts, and widen the annotation to int | None to match what callers pass. _check_marlin_supported in the same module already declares group_size: int | None and tests for None explicitly, so this follows the existing convention rather than inventing one.

Test plan

Three cases in tests/quantization/test_moe_wna16.py, one per independent read of the same field. Extended that file rather than adding a new one, per AGENTS.md.

test	site it covers	unpatched
test_wna16_oracle_handles_unset_group_size	the probe, oracle/int_wna16.py	TypeError for MARLIN
test_ct_wna16_moe_method_normalises_unset_group_size	the loading path, CompressedTensorsWNA16MoEMethod.__init__	TypeError
test_marlin_weight_prep_accepts_unset_group_size	weight prep, convert_to_wna16_moe_kernel_format	TypeError

The second one matters most: it is the path a user actually walks when serving a per-channel compressed-tensors MoE checkpoint, not a synthetic call into a helper.

platform	unpatched	with fix
B200 (sm100)	3 failed, 1 passed	4 passed
H200 (sm90)	3 failed, 1 passed	4 passed
Intel B70 (XPU)	1 failed, 1 passed, 2 skipped	2 passed, 2 skipped

Why two cases skip on XPU, and why that is not a coverage hole. select_wna16_moe_backend consults a per-platform priority list, which on XPU is [XPU]. So __init__ takes the non-Marlin branch and asserts strategy == "group" — a deliberate, pre-existing rejection of channelwise that this PR does not touch — and the Marlin probe is never reached. The weight-prep case needs gptq_marlin_repack, which has no CPU or XPU kernel. Both skip explicitly with a reason rather than passing vacuously. The probe case still discriminates on XPU because it calls _backend_incompatibility_reason with backend=MARLIN directly, bypassing the priority list — which is why the XPU column still shows a real fail-to-pass transition.

An earlier revision of these tests passed without the fix on every platform. Root cause: make_dummy_moe_config() with no arguments yields hidden_dim=1, and 1 % 128 != 0 rejects Marlin before group_size is ever compared, so the probe returned a reason without touching the None path. The shapes are now explicit (hidden_dim=4096, intermediate_size=1024) and each test asserts its own premise.

No perf or accuracy impact: this is a support-probe predicate evaluated once per layer at construction, not on any forward path. ruff check + ruff format --check clean, local pre-commit run green.

Relationship to other work

Found while investigating grouped int8 WNA16 MoE handling. This fix is independent of that question and correct either way — an incompatibility probe should not raise. The two mutually-exclusive alternatives for the int8 semantics are separate draft PRs, linked from the issue.

---

AI assistance was used (Claude Code); every changed line was reviewed and all tests above were run personally on NVIDIA B200, H200 and Intel B70 hardware.

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[afierka-intel]

Reworked after review — the previous version did not fix the reported failure.

The v1 patch normalised None inside check_moe_marlin_supports_config. That made the probe return True, so Marlin got selected, and the load then died deeper at marlin_utils.py:344 (group_size if group_size > 0 else 1) — because compressed_tensors_moe_wna16.py:477 forwards the raw weight_quant, whose group_size is still None. I confirmed that second TypeError by executing the upstream body. So v1 moved the crash later and into a less legible frame.

v2 normalises at the config-read boundary instead, in all three places that read group_size off QuantizationArgs:

• compressed_tensors_moe_wna16.py:69
• oracle/int_wna16.py:163 (support probe)
• oracle/int_wna16.py:1500 (weight prep — the second crash site)

This follows the six existing in-tree sites (compressed_tensors_wNa16.py:67 and siblings, compressed_tensors_moe.py:104). I also withdraw the precedent I cited for v1: _check_marlin_supported rejects None (returns False plus a reason) rather than coercing it, so it was not support for normalising inside the predicate.

Two more corrections to the previous description:

• "No platform checks" was false. marlin_utils.py has 6, and three gate this path: :55-56 returns only uint4 types on XPU, :368-369 returns False on ROCm, marlin_moe.py:605-607 requires is_cuda().
• The B70 row was meaningless. On XPU the WNA16 priority list is [XPU] alone, so the oracle never consults the Marlin probe — I confirmed this while retesting: MARLIN returns "not supported for this layer" before group_size is read. The three-platform table exercised one code path three times, on a platform that cannot reach the change.

Consequence: this PR needs CUDA validation with a real channel-strategy compressed-tensors int4 MoE checkpoint, loaded end to end past weight prep. The v1 unit test is withdrawn — it passed for four different normalisation targets and would also have passed with the fix reverted. I have not yet produced the replacement evidence, so treat this as not ready for review until that load succeeds.

[afierka-intel]

Superseded by vllm-project/vllm#52716, fixes vllm-project/vllm#52713. Closing the fork PR.

[afierka-intel]

Promoted upstream as vllm#53163.

Changed on the way out: rebased onto 4b7cb949a9; the int8 test commit (377ec142ae) dropped because vllm-project#52002 deleted the assert its docstring cites, so the scope is int4-only; the append-at-EOF conflict in test_moe_wna16.py against vllm-project#48918 resolved keep-both; A/B re-measured on the current diff on NVIDIA L40S (3 failed → 4 passed) and Intel B70 (1 failed → 2 passed).

Branch kept. Pre-rebase state: backup/marlin-utils-none-group-size-preRebase-20260820 = 377ec142ae.