Cortex-M backend: Add quantized s8 pad operator via CMSIS-NN

[rascani]

Summary

Add cortex_m::pad that delegates to arm_pad_s8. The operator takes pre_pad/post_pad arrays in CMSIS-NN 4D format — the AoT fusion pass converts from PyTorch's constant_pad_nd padding format, validates rank/padding constraints, and quantizes the float pad value to int8.

Pad is a shared-qspec op (input/output share quantization parameters).

Fixes #16111

Test plan

source examples/arm/arm-scratch/setup_path.sh
backends/cortex_m/test/build_test_runner.sh
pytest --config-file=backends/arm/test/pytest.ini backends/cortex_m/test/ops/test_pad.py

[pytorch-bot]

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❌ 10 New Failures

As of commit 9285d58 with merge base f4ef59f ():

NEW FAILURES - The following jobs have failed:

• Build Presets / apple (ios-simulator) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Build Presets / apple (macos) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Build Presets / apple (profiling) / build (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 123ca0d5dad6bb0bad25d0fc5bec7b68f358685a1ca98f8b5e7703fed60a1ac7 /exec failed with exit code 1
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 541d9a990ead1519926f274d70c58ec86754d03ff2f82bd89e40302c08c77807 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3247c90402d9526aef8fb827abecf49a1ef1b24484cd3d22266324b1150c6416 /exec failed with exit code 1
• Test CUDA Builds / test-models-cuda (conv1d) / linux-job (gh)
  RuntimeError: Command docker exec -t 6fcf2911a3b29867f6825ddb3912e93edc8cca374cad2a19a8c53211530a4e1b /exec failed with exit code 1
• Test CUDA Builds / test-models-cuda (linear) / linux-job (gh)
  RuntimeError: Command docker exec -t 5daf65637c321bec04358b1a4bfc5bcdf10a547016484a7f89a456cd49531c94 /exec failed with exit code 1
• trunk / test-huggingface-transformers-macos (distilbert|coreml_fp32_gpu|--quantize) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• trunk / test-huggingface-transformers-macos (llama3.2-1b|coreml_fp32_gpu|--quantize) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

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[AdrianLundell]

A few nits, but overall really nice job!

[rascani]

Failures unrelated.