Name Correction op_mean

[BujSet]

Summary

Operator and dtype selective build requires operator names to match operators in use. This changes corrects the name for mean operator. Change is similar to that in #12755.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12885

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 909fb61 with merge base 00e3f99 ():

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

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

• pull / test-phi-3-mini-runner-linux / linux-job (gh) (trunk failure)
  AttributeError: 'StaticCacheConfig' object has no attribute 'get'

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[BujSet]

@pytorchbot label "release notes: none"

[lucylq]

thanks for fixing these!