[Bugfix] Fix OOB expert_map read in moe_fused_mul_sum with invalid topk_ids

[jahnavi-yelamanchi]

Purpose

Fixes #47281. Under expert parallelism, topk_ids can contain -1 sentinels for token/expert pairs that got routed to a different EP rank (documented in fused_moe/utils.py). The moe_fused_mul_sum Triton kernel indexed expert_map with these ids without guarding against -1, so it read out of bounds — sometimes crashing with an illegal memory access (what's reported in the issue), sometimes just silently corrupting output instead. Only shows up with --enable-expert-parallel.

Fix masks the expert_map load on id_val >= 0.

Checked for dupes first and nothing else open touches this kernel or this issue.

Test Plan

New test: tests/kernels/moe/test_moe_fused_mul_sum.py, run on an L40S:

pytest tests/kernels/moe/test_moe_fused_mul_sum.py -v

Test Result

Before fix (main): 12/60 cases fail, up to 98% mismatched elements vs. torch reference, e.g.:

Mismatched elements: 126 / 128 (98.4%)
Greatest absolute difference: 2.625 (up to 0.02 allowed)
12 failed, 48 deselected

After fix: 60 passed in 24.55s

pre-commit (ruff check/format) clean on changed files.

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AI assistance (Claude Code) was used for root-cause analysis and the initial patch. I reviewed every line and ran the tests above myself.

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[claude]

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[jahnavi-yelamanchi]

Could a maintainer approve CI for this PR when you get a chance? The failing pre-run check is just the first-time-contributor gate (this is my first PR here), so the pre-commit job is being skipped rather than reporting an actual failure. I've run ruff check, ruff format --check, and git diff --check locally on the changed files and everything passes. Happy to address any feedback.

@mgoin @pavanimajety @WoosukKwon

[mergify]

Hi @jahnavi-yelamanchi, 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.