Cortex_m backend: Fix add implementation

[AdrianLundell]

• Call CMSIS-NN kernel with correct argument order and signs
• Change python implementation to reflect CMSIS-NN behaviour
• Fix scale calculations
• Remove broken broadcasting support
• Add pass to lower scalar version ops
• Remove unused definitions/ implementations in operators.py, operators.yaml and op_quantized_add.cpp

Note: arm_elementwise_add_s8 does not natively support broadcasting, so simply resizing the output tensor will not work. Enabling this in an efficient way is not straight forward, so avoid fusing these ops for now to avoid break graphs.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15100

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❌ 2 New Failures, 2 Unrelated Failures

As of commit 61dddb5 with merge base 5426918 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 179dcf6156ff0bca9546539cb5342597118ee07f5ff1278737478a7c2b41aace /exec failed with exit code 1
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest / linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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

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

• pull / unittest-editable / linux / linux-job (gh) (trunk failure)

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

[GregoryComer]

It looks like the test-mcu-models (cmake) / linux-job CI job has been failing since this PR merged. Example failure: https://github.com/pytorch/executorch/actions/runs/18869591982/job/53844591126. Can you take a look? Thanks.
CC @AdrianLundell @psiddh

[AdrianLundell]

We should remove that test IMO, it was only testing the lowering and not that we were getting reasonable results, and we have pytests which covers that now. I can create a patch for that today.