[Kernel] Add b12x FP4 MoE backend

[lukealonso]

Purpose

Builds on the optional b12x dependency, shared lazy imports, packed-storage reuse,
and warmup integration merged in #52016.

This PR adds an explicitly selected
b12x FP4 MoE backend for NVIDIA
SM120 and SM121 GPUs using vLLM's existing fused-MoE backend interfaces. It
does not introduce a new MoE abstraction.

Supported paths include:

• Native NVFP4 and MXFP4 W4A4.
• W4A16 and supported dynamic W4A8 activation modes.
• MXFP4 defaults to MXFP8 activations where supported; NVFP4 retains its
  checkpoint activation format.
• VLLM_B12X_MOE_FP4_FORCE_A16=1 forces both FP4 formats onto the BF16
  activation path.
• Tensor parallelism, warmup, reload-stable packed storage, and CUDA graph
  replay.

Expert parallelism, expert maps, EXL3, and NF3 are unsupported. b12x 1.2.6 owns
and registers the custom operator used by the backend.

Warmup supplies the complete set of serving sizes available to vLLM. b12x
deduplicates sizes that resolve to identical internal policy choices; vLLM does
not duplicate or guess those package-internal policies.

The documentation covers explicit --moe-backend b12x selection, supported
activation modes, the A16 override, and unsupported expert configurations.

This is the MoE component split from #51696, which it supersedes rather than
duplicates. Related PRs #41243 and #47577 do not provide this standalone
optional-package MoE integration.

AI assistance from OpenAI Codex was used while developing this PR. I reviewed
every changed line and am responsible for understanding and defending the
integration end-to-end.

Test Plan

Run the b12x MoE backend-selection, correctness, warmup, storage-reuse, and CUDA
graph coverage:

CUDA_VISIBLE_DEVICES=<idle-gpu> .venv/bin/python -m pytest \
  tests/kernels/moe/test_b12x.py -q

CUDA_VISIBLE_DEVICES=<idle-gpu> .venv/bin/python -m pytest \
  tests/kernels/moe/test_flashinfer_b12x_moe.py -q

.venv/bin/python -m pytest \
  tests/model_executor/test_b12x_warmup.py \
  tests/quantization/test_auto_round.py::test_inc_mxfp4_moe_method_preserves_checkpoint_packing \
  -q

.venv/bin/python -m pytest tests/quantization/test_online_mxfp4.py -q

Run pre-commit over every file changed by this PR:

mapfile -t changed_files < <(git diff --name-only origin/main...HEAD)
.venv/bin/pre-commit run --files "${changed_files[@]}"
git diff --check origin/main...HEAD

Benchmark nvidia/MiniMax-M2.7-NVFP4 on an RTX PRO 6000 Blackwell Max-Q GPU,
comparing b12x with FlashInfer CUTLASS MoE at TP2. Use five warmup and ten
measured requests.

Test Result

Focused tests against the published b12x==1.2.6 wheel:

37 b12x MoE tests passed
48 FlashInfer-b12x MoE comparison tests passed
7 shared warmup tests passed
2 AutoRound integration parametrizations passed
10 online/Quark MXFP4 tests passed; 8 skipped

All changed-file pre-commit hooks passed, and git diff --check reported no
errors.

Single-request end-to-end decode throughput; higher is better:

Path	Model/configuration	TP	Comparison backend	Comparison tok/s	b12x tok/s	Change
NVFP4 MoE	MiniMax-M2.7-NVFP4	2	FlashInfer CUTLASS MoE	132.1558	137.8276	+4.29%

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR.
• The test plan, including commands.
• The test and performance results.
• The necessary documentation update.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[github-actions]

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[mergify]

Documentation preview: https://vllm--52018.org.readthedocs.build/en/52018/

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @lukealonso.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[github-actions]

✅ @lukealonso, CI is now available for this PR.

• /ci run starts upstream CI; /amd-ci run starts AMD CI only.
• /ci retry retries failed jobs in the CI build for the current PR head. If the current head has no CI build, it starts a new CI build for the current head containing only jobs that failed in the latest earlier CI build for this PR.
• /amd-ci retry retries failed jobs in AMD CI for the current PR head. Use /amd-ci run when the current head has no AMD CI build.
• /ci cancel cancels scheduled or running CI builds for this PR branch; /amd-ci cancel does the same for AMD CI only.

[mergify]

Hi @lukealonso, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[mgoin]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84660 for commit f21c006efde7.

[lukealonso]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84741 for commit f21c006efde7.

[mgoin]

Triggered spark build

• arm image https://buildkite.com/vllm/ci/builds/84741/canvas?sid=01a01d4d-285d-4f14-b671-2372954c4c63&tab=output
• b12x test https://buildkite.com/vllm/ci/builds/84741/canvas?sid=01a01d4d-290c-4d7d-b913-e6590fbcbd7f&tab=output

[lukealonso]

/ci run

[mgoin]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84885 for commit 832fdb1493b4.