RPATH Fix for portable_lib Python Extension

[pytorchbot]

Note: This is an attempt to cherry-pick Mergen's RPATH fix from #13254 onto main. Fixes #14421. Original description below.

Problem: The _portable_lib.so Python extension built on CI couldn't find PyTorch libraries when installed locally because it had hardcoded absolute paths from
the CI build environment.

Error:
ImportError: dlopen(.../_portable_lib.cpython-311-darwin.so, 0x0002): Library not loaded: @rpath/libtorch_python.dylib
Referenced from:
.../executorch/extension/pybindings/_portable_lib.cpython-311-darwin.so Reason: tried:
'/Users/runner/work/_temp/.../torch/lib/libtorch_python.dylib' (no such file)

Root Cause: The CMake build was linking to PyTorch libraries using absolute paths from the build environment, without setting proper relative RPATHs for
runtime library resolution.

Solution: Added platform-specific relative RPATH settings to the portable_lib target in /Users/mnachin/executorch/CMakeLists.txt (lines 657-669):

• macOS: Uses @loader_path/../../../torch/lib to find PyTorch libraries relative to the .so file location
• Linux: Uses $ORIGIN/../../../torch/lib for the same purpose
• Sets both BUILD_RPATH and INSTALL_RPATH to ensure consistency

Impact: This allows the wheel-packaged _portable_lib.so to find PyTorch libraries regardless of the installation location, fixing the runtime linking issue
when using ExecutorTorch wheels built on CI.

Note: The same fix may be needed for _training_lib if it experiences similar issues.

Test Plan:

  # Build the wheel locally
  python setup.py bdist_wheel

  # create fresh conda env
  conda create -yn executorch_test_11 python=3.11.0 && conda activate executorch_test_11

  # install
  pip install ./dist/executorch-*.whl

  # Verify
  python -c "from executorch.extension.pybindings._portable_lib import _load_for_executorch; print('Success!')"

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14442

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❌ 1 New Failure, 1 Cancelled Job

As of commit c6f46e3 with merge base e0dda90 ():

NEW FAILURE - The following job has failed:

• trunk / test-arm-ootb-linux / linux-job (gh)
  RuntimeError: Command docker exec -t a81bbceec7a62067fb55f00dcec9696e4a2a055b2fd1cfc6860605b10a43f32f /exec failed with exit code 1

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

• trunk / test-models-macos-coreml (emformer_transcribe) / macos-job (gh)
  ##[error]The operation was canceled.

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