Convert pthreadpool to ExternalProject

[swolchok]

As with the previous PR #12425, I think this is needed to get EXPORT
to work.

[swolchok]

Stack from ghstack (oldest at bottom):

• Use CMake EXPORT #8954
• -> Convert pthreadpool to ExternalProject #12468
• Build XNNPACK as an ExternalProject #12425
• Format CMakeLists.txt files about to be changed in EXPORT PR #12463
• [easy] cmake-format extension/training/CMakeLists.txt #12451
• [easy] cmake-format CMakeLists.txt #12450

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12468

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 11 New Failures

As of commit 35d8319 with merge base ee9e6d9 ():

NEW FAILURES - The following jobs have failed:

• Apple / build-benchmark-app / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Apple / build-frameworks-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Apple / test-demo-ios / mobile-job (ios) (gh)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• Build Linux Wheels / pytorch/executorch / build-manywheel-py3_10-cpu (gh)
  Process completed with exit code 1.
• Build Linux Wheels / pytorch/executorch / upload / upload-manywheel-py3_10-cpu (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_executorch__3.10_cpu_x86_64
• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1f376b02a515b9f58a49fd9ae2625165f96345586b5c78a9d6b5c9b71a79edf9 /exec failed with exit code 1
• pull / test-llava-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f5a7ea3c794186496998ea9785c5122d16370446c0bae474a0f69000e798d7b8 /exec failed with exit code 2
• pull / unittest / macos / macos-job (gh)
  extension/llm/custom_ops/test_sdpa_with_kv_cache.py::SDPATestForLargeSeqLength::test_sdpa_with_cache_seq_len_130
• pull / unittest-editable / macos / macos-job (gh)
  extension/llm/custom_ops/test_sdpa_with_kv_cache.py::SDPAWithCausalTest::test_sdpa_with_cache_no_mqa_3
• trunk / test-llama-torchao-lowbit / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 2
• trunk / unittest-release / macos / macos-job (gh)
  extension/llm/custom_ops/test_sdpa_with_kv_cache.py::SDPAWithCausalTest::test_sdpa_with_cache_no_mqa_3

This comment was automatically generated by Dr. CI and updates every 15 minutes.