Add MPS support to llama runner (#3464)

Summary:
As titled. This let me easily test MPS delegate on llama runner.

Commands:

* Install ExecuTorch libraries:

```
cmake -DCMAKE_INSTALL_PREFIX=cmake-out \
        -DCMAKE_BUILD_TYPE=Debug \
        -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON \
        -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON \
        -DEXECUTORCH_BUILD_CUSTOM=ON \
	-DEXECUTORCH_BUILD_OPTIMIZED=ON \
        -DEXECUTORCH_BUILD_QUANTIZED=ON \
        -DEXECUTORCH_BUILD_XNNPACK=ON \
	-DEXECUTORCH_BUILD_MPS=ON \
	-DPYTHON_EXECUTABLE=python \
	-Bcmake-out .

cmake --build cmake-out -j9 --target install --config Debug
```
* Install llama runner
```
cmake -DCMAKE_INSTALL_PREFIX=cmake-out \
        -DCMAKE_BUILD_TYPE=Debug \
        -DEXECUTORCH_BUILD_OPTIMIZED=ON \
	-DPYTHON_EXECUTABLE=python \
	-Bcmake-out/examples/models/llama2 \
	examples/models/llama2
cmake --build cmake-out/examples/models/llama2 -j9 --config Debug
```

Pull Request resolved: #3464

Reviewed By: cccclai

Differential Revision: D56856480

Pulled By: larryliu0820

fbshipit-source-id: 91d2c4fda4fda8b42bebd49a3be9f9b9226994d2

Author: larryliu0820

examples/models/llama2/CMakeLists.txt

@@ -92,8 +92,7 @@ if(EXECUTORCH_USE_TIKTOKEN)
   # find RE2 for tokenizer
   set(ABSL_ENABLE_INSTALL ON)
   set(ABSL_PROPAGATE_CXX_STD ON)
-  set(_pic_flag
-    ${CMAKE_POSITION_INDEPENDENT_CODE})
+  set(_pic_flag ${CMAKE_POSITION_INDEPENDENT_CODE})
   set(CMAKE_POSITION_INDEPENDENT_CODE ON)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/third-party/abseil-cpp)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/third-party/re2)
@@ -169,6 +168,19 @@ if(TARGET qnn_executorch_backend)
   target_link_options_shared_lib(qnn_executorch_backend)
 endif()
 
+# MPS backend
+if(TARGET mpsdelegate)
+  list(
+    APPEND
+    link_libraries
+    mpsdelegate
+    "-framework Foundation"
+    "-weak_framework MetalPerformanceShaders"
+    "-weak_framework MetalPerformanceShadersGraph"
+    "-weak_framework Metal")
+  target_link_options_shared_lib(mpsdelegate)
+endif()
+
 # This one is needed for cpuinfo where it uses android specific log lib
 if(ANDROID)
   list(APPEND link_libraries log)