Use dynamic CPU count for -j in vendor/example scripts and docs

[ShamSaleem]

Summary

Follow-up to #20436, which replaced the hardcoded cmake --build -j parallelism in the general
build docs and test/ scripts. This PR finishes the same job for the vendor backend scripts,
example scripts, and their documentation — 65 sites across 37 files, all mechanical:

-j$(( $(nproc 2>/dev/null || sysctl -n hw.ncpu) + 1 ))

nproc on Linux, sysctl -n hw.ncpu on macOS (the mps and coreml scripts are Apple-only), and the
arithmetic degrades to -j1 if neither tool exists. "Core count + 1" is the guidance already in
docs/source/using-executorch-building-from-source.md. The pinned values being removed ranged from
-j4 to -j100, including -j64 in the Vulkan test scripts and -j100 in
tools/cmake/preset/README.md.

Two Python sites differ. extension/llm/export/quantizer_lib.py holds a shell command inside a
user-facing error string, so it takes the same shell expression. backends/mlx/test/test_utils.py
builds an argv list handed to subprocess.run with no shell, where a shell expression would reach
cmake as a literal string, so it uses f"-j{(os.cpu_count() or 1) + 1}" instead.

Deliberately out of scope: .ci/** and .github/workflows/**, where the runners are fixed-size and
the parallelism is a resource-tuning decision rather than a portability problem (cuda.yml pins
-j4, likely to bound peak memory); the -j4 in backends/mlx's READMEs and run_all_tests.py,
which is a test-worker count and not a build flag; and docs/source/archive/. Happy to take the CI
files in a separate PR if you'd like them changed.

Review order: the three groups are independent — vendor backend scripts under backends/, example
scripts and docs under examples/, then the two Python files, which are the only sites that are not
a pure token swap.

Partial fix for #10887.

Test plan

ExecuTorch does not build on my Windows host, so verification is static and per-site:

• bash -n passes on all 20 modified shell scripts.
• Every edited command line was re-run with cmake --build/make swapped for echo, confirming all
  65 sites expand to a single valid integer flag (-j17 on this 16-core machine) with zero
  expansion failures. This covers the markdown sites too, including the two lines that begin with
  && and the one with a $ prompt prefix.
• git diff normalised on the -j token shows every removed line has a matching added line, so
  nothing outside the flag changed. Every changed markdown line contains a -j token.
• Both Python files parse; the argv site renders -j17; the instruction string was extracted via
  ast and shell-expanded to confirm a user pasting it gets -j17.
• black --check reports both Python files unchanged. Line endings are unchanged (still LF) and
  git diff --check reports no whitespace errors.
• lintrunner was not run locally — it is not installed on this Windows host and is unavailable
  in my WSL environment, so CI lint is the gate for that. E501 is in the repo's flake8 ignore list,
  so the one long instruction string in quantizer_lib.py (already 165 chars before this change) is
  not a new violation.

This PR was authored with AI assistance (Claude Code); the diff and every verification step above
were reviewed by me.

cc @GregoryComer @digantdesai @cbilgin @JakeStevens @larryliu0820

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21455

• 📄 Preview Python docs built from this PR

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

❌ 2 New Failures, 1 Unclassified Failure

As of commit 871f48c with merge base d0fe35d ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 6e23ec8551e5c206092cbe9ae517030f572ac01b7553d1a5ed9b12ae4236fcd2 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 93a78f6dfa8d4b22df03202ad9d687c1042ad4c338e1bdaad6391b531bb2f0e6 /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• periodic / gather-models (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)

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

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.