Disable mean fallback op (#18815)

[mcremon-meta]

Summary:

POR models (and virtually anything else that I've seen) only use mean on floats. Removing the fallback by default saves 75.6kB in .text.

Reviewed By: DrJessop

Differential Revision: D100258757

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18815

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 3 Unrelated Failures

As of commit 11bdef7 with merge base 8ed6e85 ():

NEW FAILURES - The following jobs have failed:

• pull / android / build-android (gh)
  Process completed with exit code 2.
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv2_model

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

• pull / test-binary-size-linux-gcc / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

BROKEN TRUNK - The following jobs failed but was 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.

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[meta-codesync]

@mcremon-meta has exported this pull request. If you are a Meta employee, you can view the originating Diff in D100258757.

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