Metal backend: import qmv_impl from MLX

[manuelcandales]

This pull request extends support in the Metal backend's 4-bit quantized linear operation (aoti_torch_mps__linear_fp_act_4bit_weight) to the case where the batch size (M) is 1 and the output dimension (N) is not a multiple of 4. To achieve this, we imported the qmv_impl Metal shader from MLX.

[manuelcandales]

Stack from ghstack (oldest at bottom):

• Metal backend: fix bugs in qmv_impl #17188
• -> Metal backend: import qmv_impl from MLX #17187
• Metal backend: int4 refactor checks and logs #17186
• Metal backend: test 4-bit linear with bias #17127
• Metal backend: enable quantization in test_modules #17118
• Metal backend: support 4-bit linear #17117
• Metal backend: remove parakeet linear decomp #17116
• Metal backend: enable linear with bias #17115
• Metal backend: test modules #17076

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17187

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❌ 4 New Failures, 1 Unrelated Failure

As of commit 8ee7d60 with merge base ba6de95 ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t b7afc97a5951192b2ca3603fc045800cbf31751e2147f4021194f4a68b08b728 /exec failed with exit code 139
• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv2d.py::TestConv2d::test_fp16_conv2d
• pull / unittest-arm-backend-with-no-deps (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 0728b7143aa8e0dbc6b93ed79e7bed4e71f452306e64e86c203cd8c4224f5f46 /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 242f03543337b2858d57468f3e35c339e7b47570646cdbeb69ef30b71e127157 /exec failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

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

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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