RPATH Fix for portable_lib Python Extension (#13254)

RPATH Fix for portable_lib Python Extension

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!')"
```

Author: mergennachin

.ci/scripts/wheel/test_base.py

@@ -41,6 +41,18 @@ class ModelTest:
 
 
 def run_tests(model_tests: List[ModelTest]) -> None:
+    # Test that we can import the portable_lib module - verifies RPATH is correct
+    print("Testing portable_lib import...")
+    try:
+        from executorch.extension.pybindings._portable_lib import (  # noqa: F401
+            _load_for_executorch,
+        )
+
+        print("✓ Successfully imported _load_for_executorch from portable_lib")
+    except ImportError as e:
+        print(f"✗ Failed to import portable_lib: {e}")
+        raise
+
     # Why are we doing this envvar shenanigans? Since we build the testers, which
     # uses buck, we cannot run as root. This is a sneaky of getting around that
     # test.

CMakeLists.txt

@@ -654,6 +654,20 @@ if(EXECUTORCH_BUILD_PYBIND)
   target_compile_options(portable_lib PUBLIC ${_pybind_compile_options})
   target_link_libraries(portable_lib PRIVATE ${_dep_libs})
 
+  # Set RPATH to find PyTorch libraries relative to the installation location
+  # This goes from executorch/extension/pybindings up to site-packages, then to torch/lib
+  if(APPLE)
+    set_target_properties(portable_lib PROPERTIES
+      BUILD_RPATH "@loader_path/../../../torch/lib"
+      INSTALL_RPATH "@loader_path/../../../torch/lib"
+    )
+  else()
+    set_target_properties(portable_lib PROPERTIES
+      BUILD_RPATH "$ORIGIN/../../../torch/lib"
+      INSTALL_RPATH "$ORIGIN/../../../torch/lib"
+    )
+  endif()
+
   install(TARGETS portable_lib
           LIBRARY DESTINATION executorch/extension/pybindings
   )