Minor perf improvements to quantized mat mul shader.

[trivedivivek]

Summary: The diff includes minor performance improvements to the quantized matrix multiplication shader.

Differential Revision: D84998542

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15261

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❌ 8 New Failures, 4 Pending, 3 Unrelated Failures

As of commit ece3cf5 with merge base 5d71c9b ():

NEW FAILURES - The following jobs have failed:

• Build Windows Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128
• Build Windows Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x64
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 5c7c30b492943c7a1987c3a006ac65f2cc3a3c9fbe0b17ccd0255079eb23d423 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 2d12781c8efd96815c8e57c506c7a5fcdd1bb309fdda3a0324bff5611dbf1492 /exec failed with exit code 1
• Test CUDA Builds / export-voxtral-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 125a5ce68537aed7c988b02186f7fcddd1551cf9433ea62b92d60ee4ddc05858 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t 0fe6330b61121712d0cd577d041e7f32a4789b7884227fa3bf0a11c060ac36e4 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-weight-only / linux-job (gh)
  RuntimeError: Command docker exec -t 9382f14468aa290af110803edee850d1341a3e77a6cd6002bdef7e22d147641e /exec failed with exit code 2
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-models-linux-basic (vit, xnnpack-quantization-delegation, buck2, linux.2xlarge, executorch-u... / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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[meta-codesync]

@trivedivivek has exported this pull request. If you are a Meta employee, you can view the originating Diff in D84998542.