[multimodal] Fix pybind in nightly wheel build

Right now after
```
pip install executorch==1.1.0.dev20251001 --extra-index-url
https://download.pytorch.org/whl/nightly
```

Then run

```
python -c "from executorch.extension.llm.runner import MultimodalRunner"
```

I'm getting this error:

```
(executorch) [[email protected] /data/users/larryliu/executorch (main)]$ python -c "from executorch.extension.llm.runner import MultimodalRunner"
Traceback (most recent call last):
  File "/home/larryliu/.conda/envs/executorch/lib/python3.11/site-packages/executorch/extension/llm/runner/__init__.py", line 16, in <module>
    from executorch.extension.llm.runner._llm_runner import (  # noqa: F401
ImportError: libtorch_python.so: cannot open shared object file: No such file or directory

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/larryliu/.conda/envs/executorch/lib/python3.11/site-packages/executorch/extension/llm/runner/__init__.py", line 29, in <module>
    raise RuntimeError(
RuntimeError: LLM runner is not installed. Please build ExecuTorch from source with EXECUTORCH_BUILD_PYBIND=ON
```

This PR should fix it by properly point to the installation path of
pytorch.

Author: larryliu0820

extension/llm/runner/CMakeLists.txt

@@ -106,10 +106,26 @@ if(EXECUTORCH_BUILD_PYBIND)
   )
   if(APPLE)
     set(RPATH "@loader_path/../../pybindings")
+    set(TORCH_RPATH "@loader_path/../../../torch/lib")
   else()
     set(RPATH "$ORIGIN/../../pybindings")
+    set(TORCH_RPATH "$ORIGIN/../../../torch/lib")
   endif()
-  set_target_properties(_llm_runner PROPERTIES INSTALL_RPATH ${RPATH})
+  # Set RPATH to find PyTorch libraries relative to the installation location
+  # This goes from executorch/extension/pybindings up to site-packages, then to
+  # torch/lib. This is copied from root level CMakeLists.txt
+  if(APPLE)
+    set_target_properties(
+      _llm_runner PROPERTIES BUILD_RPATH "${TORCH_RPATH}"
+                             INSTALL_RPATH "${RPATH};${TORCH_RPATH}"
+    )
+  else()
+    set_target_properties(
+      _llm_runner PROPERTIES BUILD_RPATH "${TORCH_RPATH}"
+                             INSTALL_RPATH "${RPATH};${TORCH_RPATH}"
+    )
+  endif()
+
   # Add include directories
   target_include_directories(
     _llm_runner PRIVATE ${_common_include_directories} ${TORCH_INCLUDE_DIRS}