Revert "Unbreak optimized kernels buck build (and check it in unittest-buck) (#9159)"

[swolchok]

This reverts commit 70d4427.

[swolchok]

Stack from ghstack (oldest at bottom):

• Revert "Revert "Split & remove extension_parallel (#8983)"" #9190
• Revert "Split & remove extension_parallel (#8983)" #9189
• Revert "Make serial parallel_for "polyfill" iterate backwards in debug builds (#9044)" #9188
• -> Revert "Unbreak optimized kernels buck build (and check it in unittest-buck) (#9159)" #9187

[pytorch-bot]

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❌ 2 New Failures, 39 Pending

As of commit 8166752 with merge base 705f100 ():

NEW FAILURES - The following jobs have failed:

• Check Labels / Check labels (gh)
  RuntimeError: Error checking labels: PR does not have required labels
• trunk / test-static-llama-ane / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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