Add top-level CMake extensions target (pytorch#12696)

Add a top level extensions CMake target, which includes all configured
extensions. I validate this change by building a simple runner using the
executorch_extensions target and verifying that it was able to build and
run using the module and tensor extension. Once the changes land in
ExecuTorch, this will be long-term validated in executorch-examples CI.

This is done in the context of top-level CMake targets, tracked in
pytorch#12293.

Author: GregoryComer

CMakeLists.txt

@@ -383,6 +383,9 @@ add_library(executorch_no_prim_ops ALIAS executorch_core)
 # A list of all configured backends.
 set(_executorch_backends "")
 
+# A list of all configured extensions.
+set(_executorch_extensions "")
+
 target_link_libraries(executorch_core PRIVATE program_schema)
 if(ANDROID)
   target_link_libraries(executorch_core PUBLIC log)
@@ -579,6 +582,7 @@ endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_APPLE)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/apple)
+  list(APPEND _executorch_extensions apple_extension)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER)
@@ -589,6 +593,7 @@ if(EXECUTORCH_BUILD_EXTENSION_DATA_LOADER)
     FILES_MATCHING
     PATTERN "*.h"
   )
+  list(APPEND _executorch_extensions extension_data_loader)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_EVALUE_UTIL)
@@ -603,6 +608,7 @@ endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/flat_tensor)
+  list(APPEND _executorch_extensions extension_flat_tensor)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_MODULE)
@@ -613,6 +619,7 @@ if(EXECUTORCH_BUILD_EXTENSION_MODULE)
     FILES_MATCHING
     PATTERN "*.h"
   )
+  list(APPEND _executorch_extensions extension_module_static)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_LLM)
@@ -632,14 +639,17 @@ if(EXECUTORCH_BUILD_EXTENSION_LLM)
         ${ORIGINAL_CMAKE_POSITION_INDEPENDENT_CODE_FLAG}
     )
   endif()
+  list(APPEND _executorch_extensions tokenizers)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_LLM_RUNNER)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/llm/runner)
+  list(APPEND _executorch_extensions extension_llm_runner)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_LLM_APPLE)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/llm/apple)
+  list(APPEND _executorch_extensions extension_llm_apple)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_RUNNER_UTIL)
@@ -650,10 +660,12 @@ if(EXECUTORCH_BUILD_EXTENSION_RUNNER_UTIL)
     FILES_MATCHING
     PATTERN "*.h"
   )
+  list(APPEND _executorch_extensions extension_runner_util)
 endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_TENSOR)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/tensor)
+  list(APPEND _executorch_extensions extension_tensor)
 endif()
 
 if(EXECUTORCH_BUILD_PTHREADPOOL AND EXECUTORCH_BUILD_CPUINFO)
@@ -756,6 +768,7 @@ endif()
 
 if(EXECUTORCH_BUILD_EXTENSION_TRAINING)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/extension/training)
+  list(APPEND _executorch_extensions extension_training)
 endif()
 
 if(EXECUTORCH_BUILD_KERNELS_LLM)
@@ -778,25 +791,22 @@ if(EXECUTORCH_BUILD_VGF)
   list(APPEND _executorch_backends vgf_backend)
 endif()
 
-
 # Top-level interface targets.
-add_library(executorch_backends INTERFACE)
-add_library(executorch::backends ALIAS executorch_backends)
 
 # A target containing all configured backends.
+add_library(executorch_backends INTERFACE)
+add_library(executorch::backends ALIAS executorch_backends)
 target_link_libraries(executorch_backends INTERFACE ${_executorch_backends})
 
-install(
-  TARGETS executorch_backends
-  INCLUDES
-  DESTINATION ${_common_include_directories}
-)
+# A target containing all configured extensions.
+add_library(executorch_extensions INTERFACE)
+add_library(executorch::extensions ALIAS executorch_extensions)
+target_link_libraries(executorch_extensions INTERFACE ${_executorch_extensions})
 
 if(EXECUTORCH_BUILD_EXECUTOR_RUNNER)
   # Baseline libraries that executor_runner will link against.
   set(_executor_runner_libs executorch extension_evalue_util
-                            extension_runner_util gflags
-                            executorch_backends
+                            extension_runner_util gflags executorch_backends
   )
 
   if(EXECUTORCH_BUILD_KERNELS_OPTIMIZED)