[Bugfix] compressed-tensors: restore int8 grouped WNA16 MoE support

[y0hnn]

Purpose

Restores the fix from #47154, which was lost in #44570.

#47154 dropped the group_size == -1 assert from the int8 branch of
CompressedTensorsWNA16MarlinMoEMethod so that checkpoints with group-quantized int8
experts could load. #44570 then merged that class into
CompressedTensorsWNA16MoEMethod, deleting the fixed file and keeping the other
class's version of the branch, which still carried the assert.

Net effect: any compressed-tensors checkpoint whose MoE experts are not all quantized
to the same width fails to load again, on 0.27.0 onwards.

tag	assert self.group_size == -1
v0.25.1	absent
v0.26.0	absent
v0.27.0	present
v0.27.1	present
main	present

Test Plan

Repro is poolside/Laguna-XS-2.1-INT4, the checkpoint #47154 was written for. Its
experts are mixed: int4 g128 on layers 1-30, int8 g128 on layers 31-39 (we are testing with A4500 GPUs).

vllm serve poolside/Laguna-XS-2.1-INT4 --tensor-parallel-size 2 --max-model-len 131072

Layers 1-30 build, layer 31 hits assert 128 == -1. No download needed to see it — the
constructor alone is enough:

from compressed_tensors.quantization import (
    QuantizationArgs, QuantizationStrategy, QuantizationType)
from vllm.model_executor.layers.quantization.compressed_tensors.compressed_tensors_moe.compressed_tensors_moe_wna16 import (  # noqa: E501
    CompressedTensorsWNA16MoEMethod)

CompressedTensorsWNA16MoEMethod(
    QuantizationArgs(num_bits=8, type=QuantizationType.INT,
                     strategy=QuantizationStrategy.GROUP, symmetric=True,
                     dynamic=False, group_size=128),
    None,
    moe_config,   # any FusedMoEConfig
)

Test Result

Before, on 0.27.0 (2x RTX A4500, TP=2). This is the whole output — the assert has no
message and no worker traceback surfaces, even at VLLM_LOGGING_LEVEL=DEBUG:

INFO  [gpu_model_runner.py:5308] Starting to load model poolside/Laguna-XS-2.1-INT4...
INFO  [compressed_tensors_moe.py:132] Using CompressedTensorsWNA16MoEMethod
INFO  [int_wna16.py:297] Using 'MARLIN' WNA16 MoE backend.
ERROR [multiproc_executor.py:912] AssertionError
ERROR [core.py:1349] Exception: WorkerProc initialization failed due to an exception in a background process.

The MARLIN/WNA16 lines are info_once from layer 1 and are misleading: the assert
runs during layer construction, before backend selection and before any weights are
read. No flag works around it — --moe-backend, --enable-expert-parallel,
--enforce-eager and the VLLM_MOE / VLLM_MARLIN_* env vars are all evaluated
later.

After, with the line removed: the assert no longer fires and construction proceeds to
backend selection.

[claude]

Claude Code Review

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