Give targets that link torch_cpu_library torch's compile settings back

Linking the torch target does more than link. It also adds torch's
include directories as system headers and TORCH_CXX_FLAGS to every
target that links it. torch_cpu_library, which these targets now link
instead so that a CPU-only torch can load them, carries neither.

So two things changed for those targets. torch/csrc/api/include became a
normal include directory, so warnings inside torch's headers are now
reported as if they were our code. And a TORCH_CXX_FLAGS set at
configure time no longer reached them. For example, compiling
kernels/portable/cpu/op_allclose.cpp with -Wall -Wextra -Werror went
from no errors to 25 errors, all inside torch's headers. Default builds
were not affected.

Add a small helper, executorch_target_torch_compile_settings, that gives
a target those two settings, and call it for each target that links
torch_cpu_library.

Author: shoumikhin

CMakeLists.txt

@@ -1461,6 +1461,7 @@ if(EXECUTORCH_BUILD_PYBIND)
         aten_bridge PRIVATE torch_cpu_library c10 executorch extension_tensor
       )
     endif()
+    executorch_target_torch_compile_settings(aten_bridge)
   endif()
 
   # pybind portable_lib

extension/llm/custom_ops/CMakeLists.txt

@@ -235,6 +235,7 @@ if(EXECUTORCH_BUILD_KERNELS_LLM_AOT)
     custom_ops_aot_lib PUBLIC cpublas torch_cpu_library extension_tensor
                               extension_threadpool
   )
+  executorch_target_torch_compile_settings(custom_ops_aot_lib)
   if(EXECUTORCH_BUILD_KERNELS_LLM_QUANTIZED_MOE_OPTIMIZED)
     target_compile_definitions(
       custom_ops_aot_lib PRIVATE EXECUTORCH_QUANTIZED_MOE_USE_TORCHAO=1

extension/llm/runner/CMakeLists.txt

@@ -127,6 +127,7 @@ if(EXECUTORCH_BUILD_PYBIND)
     _llm_runner PRIVATE extension_llm_runner tokenizers::tokenizers
                         portable_lib ${TORCH_PYTHON_LIBRARY} torch_cpu_library
   )
+  executorch_target_torch_compile_settings(_llm_runner)
   executorch_target_link_shared_runtime(_llm_runner)
 
   set_target_properties(

extension/training/CMakeLists.txt

@@ -96,6 +96,7 @@ if(EXECUTORCH_BUILD_PYBIND)
            -fexceptions>
   )
   target_link_libraries(_training_lib PRIVATE ${_pybind_training_dep_libs})
+  executorch_target_torch_compile_settings(_training_lib)
 
   if(EXECUTORCH_BUILD_SHARED
      AND EXECUTORCH_BUILD_XNNPACK

tools/cmake/Codegen.cmake

@@ -336,6 +336,7 @@ function(gen_custom_ops_aot_lib)
   include_directories(${TORCH_INCLUDE_DIRS})
   # torch adds libtorch_cuda with a CUDA build of PyTorch; torch_cpu adds MKL.
   target_link_libraries(${GEN_LIB_NAME} PRIVATE torch_cpu_library)
+  executorch_target_torch_compile_settings(${GEN_LIB_NAME})
 
   executorch_target_link_options_shared_lib(${GEN_LIB_NAME})
   if(TARGET portable_lib)

tools/cmake/Utils.cmake

@@ -280,6 +280,16 @@ macro(find_package_torch)
   endif()
 endmacro()
 
+# Compiles target_name the way linking torch would: torch's include directories
+# as system headers and TORCH_CXX_FLAGS. A target that links torch_cpu_library
+# instead of torch gets neither from it.
+function(executorch_target_torch_compile_settings target_name)
+  target_include_directories(
+    ${target_name} SYSTEM PRIVATE ${TORCH_INCLUDE_DIRS}
+  )
+  target_compile_options(${target_name} PRIVATE ${TORCH_CXX_FLAGS})
+endfunction()
+
 # Modify ${targetName}'s INTERFACE_INCLUDE_DIRECTORIES by wrapping each entry in
 # $<BUILD_INTERFACE:...> so that they work with CMake EXPORT.
 function(executorch_move_interface_include_directories_to_build_time_only