Metal backend: fix bugs in qmv_impl

[manuelcandales]

Fixes two bugs in qmv_impl (also submitted PR 3096 to MLX)

• Fix 1: Changed qdot to qdot_safe when handling remaining elements in the N < 8 case
  Notice that in qmv_impl, there are two code paths based on whether N < 8:

  • When N >= 8: Correctly uses qdot_safe for remaining elements
  • When N < 8: Incorrectly used qdot instead of qdot_safe for remaining elements (fixed in this PR)

• Fix 2: Changed in_vec_size_g from floor division to ceiling division to correctly handle cases where K is not a multiple of group size

[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]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17188

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❌ 2 New Failures, 1 Pending, 1 Unrelated Failure

As of commit 9966d37 with merge base ba6de95 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-arm-backend-with-no-deps (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t e58dcce4c0f066d3ed6238af151d194c0862de01bb1b3a05c06add9335e61013 /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t a7ad54e9a87615864ee414778888d069eda2d0c5ab39982d8866ac7a37e13e24 /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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