Disable torchao kernels by default

[pytorchbot]

This PR disables building torchao kernels from source by default.

To enable the builds, a user needs to explicitly define the environment variable USE_CPP=1:

EXECUTORCH_BUILD_TORCHAO=1 python install_executorch.py

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12794

• 📄 Preview Python docs built from this PR

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

❌ 3 New Failures, 4 Cancelled Jobs

As of commit 70173c5 with merge base de24489 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for docs/source/llm/run-on-ios.md:
• pull / android / build-llm-demo / linux-job (gh)
  RuntimeError: Command docker exec -t 939fcffc9fdca797a4a689e19161ad7e65789dd801a54da5aa01a5c2c27a5fe8 /exec failed with exit code 2
• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 98ae8181bb17a94681e20362d94de2e1d71f5da2c02e3759d5f02bffa9e230c0 /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / test-models-linux (mobilebert, portable, linux.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.
• pull / test-models-linux (mobilebert, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.
• trunk / test-models-linux-aarch64 (mobilebert, portable, linux.arm64.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.
• trunk / test-models-linux-aarch64 (mobilebert, xnnpack-quantization-delegation, linux.arm64.2xlarge) / linux-job (gh)
  ##[error]The operation was canceled.

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