[cadence] Run build/test on fork exports and surface per-job checks on the PR

[aliafzal]

Summary

The Cadence Build & Test workflow ran its credentialed build/test jobs only on same-repo PRs. On Meta-exported fork PRs (labeled meta-exported + CLA Signed) the cpu-build and cpu-test jobs were excluded from the pull_request_target path, and the jobs that did run there did not appear as checks on the PR because pull_request_target check runs attach to the base SHA.

This change:

• Routes cpu-build and cpu-test through the same gate as hifi-build/vision-build, so all four credentialed jobs run on fork exports (and skip cleanly on non-exported fork PRs). Their checkout ref now also honors pull_request_target.
• Adds export-checks-start / export-checks-finish, which mirror per-job check runs onto the PR head SHA via the Checks API, so each cadence job shows up as its own check on the export PR (in-progress then success/failure/skipped), each linking to its job logs.

Same-repo PRs continue to run via pull_request and report their checks natively; the new mirror jobs only run on the pull_request_target path.

Test plan

• Same-repo PR (this one): pull_request runs gate + cpu-build/cpu-test/hifi-build/vision-build with native checks.
• Fork export: once meta-exported + CLA Signed are applied, the pull_request_target run executes the four jobs with credentials and posts cadence (export) / <job> checks onto the PR head.

Authored with Claude Code.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20374

• 📄 Preview Python docs built from this PR

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

❌ 4 New Failures, 2 Unrelated Failures

As of commit 673fc64 with merge base 24d8bab ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 1bbae669f617642a1a6a7acd853facefcb9b1fbdda994a355dc71f2305b801e3 /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t fc1da6ebe8ef8278f77cbce6a32e7555f572a3c1821ce5ee07b56b54c3508b44 /exec failed with exit code 1
• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 84f362c6903a6552cf533d70af4a7c9c09d3ea5ab3927fefac16e2b8d6e55881 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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

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

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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[aliafzal]

Superseded by #20388 (all cadence jobs moved to pull_request_target).