[codex] Fix GLM ModelOpt NVFP4 A16 force path

[voipmonitor]

Summary

This patch makes the GLM ModelOpt NVFP4 B12X MoE wrapper actually honor
B12X_MOE_FORCE_A16=1, and routes the forced W4A16 path through B12X's native
ModelOpt W4A16 prepare API instead of the packed serving repack path.

Root Cause

The vLLM wrapper always passed an explicit quant_mode="nvfp4" for NVFP4
checkpoints. That bypassed B12X's default_moe_quant_mode() env handling, so
setting B12X_MOE_FORCE_A16=1 did not change the actual MoE quant mode.

After making the env force effective, the default packed ModelOpt NVFP4 W4A16
prepare path produced broken GLM numerics. Prefill KLD against the BF16 reference
jumped from the normal ~0.08 range to ~8-9. The native ModelOpt W4A16 prepare
path was numerically sane, so this patch uses that path for forced A16 on
ModelOpt NVFP4.

What Changed

• Add local env flag parsing for B12X_MOE_FORCE_A16 in the vLLM B12X MoE
  wrapper.
• When the checkpoint is ModelOpt NVFP4 and the env flag is enabled, return
  quant_mode="w4a16" and log the forced mode once.
• Use w13_layout="w13" for ModelOpt NVFP4 W4A16, matching the native ModelOpt
  W4A16 contract.
• Prepare forced ModelOpt NVFP4 A16 weights with
  prepare_w4a16_modelopt_native_weights() instead of the packed W4A16 repack
  helper.

Validation

• git diff --check
• python3 -m py_compile vllm/model_executor/layers/fused_moe/b12x_moe.py
• Built image:
  voipmonitor/vllm:cu132-vllm611a842-b12xf9226c-a16nativew4a16-20260606
• GLM-5.1 NVFP4 TP8/DCP1 prefill KLD, B12X_MOE_FORCE_A16=1, native ModelOpt
  W4A16:
  • Mean KLD: 0.052297 over 2047 positions
• Same setup with the packed W4A16 path was numerically broken:
  • packed default: Mean KLD: 8.613591
  • packed + attempted scale folding: Mean KLD: 9.499678
  • packed + w13_layout="w13": Mean KLD: 9.499678
• Runtime smoke on patched image:
  • launched GLM-5.1 on port 5329
  • log confirmed B12X_MOE_FORCE_A16=1 forcing B12X MoE quant_mode=w4a16
  • API startup completed
  • /mnt/test.py --port 5329 --model GLM-5.1 --max-tokens 80: TTFT 0.69s,
    generation-only 76.13 tok/s, CJK 0

Caveat

The native ModelOpt W4A16 path fixes correctness but is not yet the performance
fix for prefill. The KLD harness with prompt_logprobs=-1 reproduced ~64 input
tok/s for a 2048-token prompt. The next optimization target is the B12X packed
ModelOpt NVFP4 W4A16 repack/runtime contract, which is currently fast but
numerically wrong for GLM.

[github-actions]

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