Build custom portable ops

[lucylq]

Summary

Build custom portable ops, allow for easy import in python.

Test plan

Confirm memory planning can be tested in python

python -m unittest exir.tests.test_memory_planning

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17642

• 📄 Preview Python docs built from this PR

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

❌ 6 New Failures, 1 Cancelled Job, 1 Unrelated Failure

As of commit 2e9bcff with merge base c4e325d ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (pybind) / build (gh)
• Lint / link-check / lint-file-size (gh)
• Lint / lintrunner-mypy (gh)
• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3b86cc1b46c9ae35050da09cfde0ca1b2e0aa58079ab6312e2e42a37deffdb7b /exec failed with exit code 2
• pull / test-vulkan-operators-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1973288df4b59b97476c94958f874526c7e5ceaf678fbaf63870435a208122ba /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t f6628508200a86d1df0066da460cb013509ad0bcc31547e40dcfdcc2f06b0858 /exec failed with exit code 3

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

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

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

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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