Arm backend: support mean.default

[Erik-Lundell]

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15363

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❌ 8 New Failures, 3 Unrelated Failures

As of commit 31ce12b with merge base f7ca57e ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 493f39f4818ebd6672b3f6e361b43d86f7d41da1a7c82b0e4619ff1f59cd0952 /exec failed with exit code 1
• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 7ae60fc163e813b11effc56ba41cf80f110861c1820e3275664801edca768f44 /exec failed with exit code 1
• Test CUDA Builds / export-gemma3-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 4274b89d4dd9afdd6f5de589dbb4f7865f6a7ab7b707f9b21d40bd922343dafc /exec failed with exit code 2
• Test CUDA Builds / export-gemma3-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t 40dab6a79b56b7caf5197f3b0c8cb3980c1889b21d3f6e23616b4e1fd1521bcc /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-non-quantized / linux-job (gh)
  RuntimeError: Command docker exec -t 4843c794f9819a2dea8f9622fc18a795d8a2a70cce741b61db9c824530d5a13f /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-tile-packed / linux-job (gh)
  RuntimeError: Command docker exec -t cb3bcc9fa0ebf9b5acf92b5aae67c97ca8483e78f5b630abed24dd9d7b7ad247 /exec failed with exit code 2
• Test CUDA Builds / export-voxtral-cuda-quantized-int4-weight-only / linux-job (gh)
  RuntimeError: Command docker exec -t c64929d9634c4dba6c4847d4725c1d038d50b456be09f4efccf7731d337f7ea5 /exec failed with exit code 2
• 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 jobs failed but were likely due to flakiness present on trunk:

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

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

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

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

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[Sebastian-Larsson]

Unrelated CI failures.