Metal backend: int4 refactor checks and logs

[manuelcandales]

This pull request improves the aoti_torch_mps__linear_fp_act_4bit_weight function in the Metal backend by adding comprehensive input validation for all tensors and enhancing debug logging. The changes ensure that tensor shapes, data types, and memory layouts are strictly checked before kernel execution, reducing the risk of runtime errors and making debugging easier.

Input validation improvements:

• Added strict checks for the shape, data type, and contiguity of the A, B, S, and Z tensors, including alignment requirements for dimensions and packed formats. Errors now provide detailed messages and return early if validation fails.
• Ensured that the B tensor's packed shape matches the expected calculation for 4-bit weights and that both S and Z tensors are 2D, have the correct leading dimension, and are contiguous.

Debug logging enhancements:

• Expanded debug logs to include detailed tensor shapes and strides for all relevant tensors after validation, improving traceability.
• Added log statements to indicate which kernel is being dispatched, aiding in kernel selection debugging. [1] [2]

[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/17186

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❌ 3 New Failures, 2 Unrelated Failures

As of commit c16dc59 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 a478603f44359c95dcf6a7505a38b3ee5907c0970b7ab44572e04e7ce4c06c54 /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t e6fb340a5f711eb4b84595cac505b51dfddcd07c385dc86c0aad5efcd8fd7edb /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux (ic4, xnnpack-quantization-delegation, linux.4xlarge.memory) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-phi-3-mini-runner-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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