NXP backend: Fix mean.dim delegation and tests.

[MartinPavella]

Summary

This PR updates the delegation condition for the aten,mean,dim operator to better reflect the requirements of Neutron.

Test plan

Unit tests provided.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14581

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❌ 5 New Failures, 4 Unrelated Failures

As of commit c5280bb with merge base b3f3111 ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t 15a0cf56273bcbe267bbd7faf5a7affb0a28a12cd4e8c5af18b20b303d538a47 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t e2b916bd629777a0d5370a863c4a6e32fdcfde4f7b7db639aec528bc7b14aaf6 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t a80a7b615b49e35ce4a2eacc9a14f756540178965509c495a028f5df32bc757b /exec failed with exit code 1
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 6ab152cddb8d5c1470c9ecc09986cc59455d1296c5df0bec25fbcd37e94d80a6 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t fa85aa3983c533a9705d8d56eece2b6258bc8108500844aef556c6e370f9a0d1 /exec failed with exit code 2

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

• pull / unittest-editable / macos / macos-job (gh) (similar failure)
  exir/tests/test_quant_fusion_pass.py::TestQuantFusionPass::test_embedding_torchao

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

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

• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
• pull / test-setup-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest / macos / macos-job (gh) (trunk failure)
  exir/tests/test_quant_fusion_pass.py::TestQuantFusionPass::test_embedding_torchao

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[MartinPavella]

Only the last commit is relevant. Once another PR is merged, the commit will be re-based.

[github-actions]

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