Disable mean fallback op

[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.

Differential Revision: D100258757

[pytorch-bot]

🔗 Helpful Links

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

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❗ 1 Active SEVs

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• Rolling out OSDC (ARC) runners on pull workflow for PyTorch trunk commits

❌ 3 New Failures, 2 Unrelated Failures

As of commit 15ef4bc with merge base 36e8ed9 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner (gh)
• pull / test-samsung-models-linux / linux-job (gh)
  test_w2l_fp16
• pull / unittest / macos / macos-job (gh)
  examples/models/llama3_2_vision/preprocess/test_preprocess.py::TestImageTransform::test_preprocess4

BROKEN TRUNK - The following jobs failed but were 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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