NXP backend: Resolve mean.dim format related issue.

[MartinPavella]

Summary

The existing mean.dim support in NXP backend failed to consider the format related implications of the operators
dimension reducing ability". This PR correctly identifies these edge cases and handles conversion to Neutron IR accordingly.

Test plan

Unit tests provided.

cc @robert-kalmar @JakeStevens @digantdesai @rascani

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20218

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❌ 3 New Failures, 2 Unrelated Failures

As of commit e5ac69e with merge base c37b6c3 ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / hifi-build / hifi4 (gh)
  Input required and not supplied: aws-region
• Cadence Build & Test / vision-build / vision (gh)
  Input required and not supplied: aws-region
• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 59db8cc0cebe78aa77451049e50ea9c5e0473eeb5bd280e83c66e2d0c0fd2fd0 /exec failed with exit code 1

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

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

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh) (trunk failure)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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[MartinPavella]

@novak-vaclav please feel free to have a look.

[MartinPavella]

The failing checks seem unrelated and the internal CI is passing. MErging.