Cortex-M backend: Support standalone clamp-type activations

[xingguo01]

• Add support for quantized clamp-type activations in the Cortex-M pipeline by canonicalizing relu/hardtanh/clamp to quantized aten.clamp.default for standalone int8 paths
• Extend activation fusion to cover max_pool2d.

@freddan80 @per @zingo @oscarandersson8218 @digantdesai @Sebastian-Larsson @AdrianLundell @psiddh

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18767

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 3 New Failures, 2 Cancelled Jobs, 2 Unrelated Failures

As of commit 55edd9e with merge base e109ac8 ():

NEW FAILURES - The following jobs have failed:

• 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_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t d23addaf87fe26d3f743b68a0e2425779df14e7f3b2a8e71ee2308e34be476b2 /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Check Labels (gh)
• periodic (gh)

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

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

This comment was automatically generated by Dr. CI and updates every 15 minutes.