Link QNN backend to pybinding lib when built (#13467)

This change makes it possible to run QNN models with pybindings, when
enabled. It does still require the environment setup described in the
docs.

I validated this change locally by running install_executorch.sh with
CMAKE_ARGS="-DEXECUTORCH_BUILD_QNN=ON", and then lowering and running a
simple add model. The CI set up further in this stack also leverage this
feature and test it extensively.

Author: GregoryComer

.ci/scripts/test_backend_linux.sh

@@ -25,6 +25,6 @@ source .ci/scripts/setup-vulkan-linux-deps.sh
 EXTRA_BUILD_ARGS="-DEXECUTORCH_BUILD_VULKAN=ON"
 
 # We need the runner to test the built library.
-PYTHON_EXECUTABLE=python CMAKE_ARGS="$EXTRA_BUILD_ARGS" .ci/scripts/setup-linux.sh --build-tool cmake --build-mode Release
+PYTHON_EXECUTABLE=python CMAKE_ARGS="$EXTRA_BUILD_ARGS" .ci/scripts/setup-linux.sh --build-tool cmake --build-mode Release --editable true
 
 python -m executorch.backends.test.suite.runner $SUITE --flow $FLOW --report "$ARTIFACT_DIR/test_results.csv"

CMakeLists.txt

@@ -817,6 +817,10 @@ if(EXECUTORCH_BUILD_PYBIND)
     list(APPEND _dep_libs openvino_backend)
   endif()
 
+  if(EXECUTORCH_BUILD_QNN)
+    list(APPEND _dep_libs qnn_executorch_backend)
+  endif()
+
   if(EXECUTORCH_BUILD_XNNPACK)
     # need to explicitly specify XNNPACK and xnnpack-microkernels-prod here
     # otherwise uses XNNPACK and microkernel-prod symbols from libtorch_cpu