[executorch][cuda] Optimize short-query INT4 matvec kernels

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [executorch][cuda] Optimize short-query INT5 matvec kernels #21458
• [executorch][cuda] Optimize short-query INT6 matvec kernels #21457
• -> [executorch][cuda] Optimize short-query INT4 matvec kernels #21456

Add compile-time M=2/3/4 fused short-query kernels while preserving the existing M=1 path. The shared ROWS template was reviewed and benchmarked by KernelAgent; KA found no faster safe rewrite, so the validated template is retained.

A100 kernel microbenchmarks versus main:

• M1: 0.10226 -> 0.10228 ms (performance neutral; no regression)
• M2: 0.196 -> 0.127 ms (1.54x)
• M3: 0.289 -> 0.159 ms (1.82x)
• M4: 0.383 -> 0.196 ms (1.95x)

Correctness tests cover M=1,2,3,4 with three randomized seeds. ptxas reports zero spills.

Differential Revision: D114032326

[pytorch-bot]

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❌ 3 New Failures, 1 Unclassified Failure

As of commit c5a07ba with merge base e47f373 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
  >>> Lint for backends/cuda/runtime/shims/tests/CMakeLists.txt:
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 83784fddf0f5adb637cb65ffedb45b3e6e1e5dadf0138101d1438bee043ca0aa /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 10f7a521ce8547e56eee5dde1f6a00d8b9a89667f9dda1a4aac1057689461875 /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• Test CUDA Builds / export-model-cuda-artifact (SocialLocalMobile, Qwen3.5-35B-A3B-HQQ-INT4, quantized-int4-tile-packed) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /__w/_temp/exec_script failed with exit code 1

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