Define PYTHON_EXECUTABLE only once in cmake (#10911)

### Summary
There is no need to redefine this over and over again. Do it once at the
root. Lmk if I'm missing something:

```
$ rg -t cmake -g '!examples' "resolve_python"
$ rg -t cmake -g '!examples' "NOT PYTHON_EXECUTABLE"
$ rg -t cmake -g '!examples' "python3"
$ rg -t cmake -g '!examples' "python\s"
```

### Test plan

CI


cc @larryliu0820

Author: jathu

CMakeLists.txt

@@ -48,6 +48,9 @@ project(executorch)
 # MARK: - Start EXECUTORCH_H12025_BUILD_MIGRATION --------------------------------------------------
 
 include(${PROJECT_SOURCE_DIR}/tools/cmake/common/preset.cmake)
+include(${PROJECT_SOURCE_DIR}/tools/cmake/Utils.cmake)
+include(CMakeDependentOption)
+include(ExternalProject)
 
 if(NOT CMAKE_CXX_STANDARD)
   set(CMAKE_CXX_STANDARD 17)
@@ -64,10 +67,14 @@ if(NOT CMAKE_BUILD_TYPE)
 endif()
 announce_configured_options(CMAKE_BUILD_TYPE)
 
+if(NOT PYTHON_EXECUTABLE)
+  resolve_python_executable()
+endif()
+announce_configured_options(PYTHON_EXECUTABLE)
+
 announce_configured_options(CMAKE_CXX_COMPILER_ID)
 announce_configured_options(CMAKE_TOOLCHAIN_FILE)
 announce_configured_options(BUCK2)
-announce_configured_options(PYTHON_EXECUTABLE)
 
 load_build_preset()
 include(${PROJECT_SOURCE_DIR}/tools/cmake/preset/default.cmake)
@@ -77,10 +84,6 @@ print_configured_options()
 
 # MARK: - End EXECUTORCH_H12025_BUILD_MIGRATION ----------------------------------------------------
 
-include(tools/cmake/Utils.cmake)
-include(CMakeDependentOption)
-include(ExternalProject)
-
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 # Setup RPATH.
@@ -256,11 +259,6 @@ if(EXECUTORCH_BUILD_TESTS)
   include(CTest)
 endif()
 
-if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-endif()
-message(STATUS "Using python executable '${PYTHON_EXECUTABLE}'")
-
 # TODO(dbort): Fix these warnings and remove this flag.
 set(_common_compile_options -Wno-deprecated-declarations -fPIC)
 

backends/apple/mps/CMakeLists.txt

@@ -18,10 +18,6 @@ endif()
 
 include(${EXECUTORCH_ROOT}/tools/cmake/Utils.cmake)
 
-if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-endif()
-
 set(_common_compile_options -Wno-deprecated-declarations)
 set(_common_include_directories ${EXECUTORCH_ROOT}/..)
 

backends/cadence/CMakeLists.txt

@@ -30,10 +30,6 @@ add_compile_definitions(C10_USING_CUSTOM_GENERATED_MACROS)
 if(EXECUTORCH_CADENCE_CPU_RUNNER)
   include(${EXECUTORCH_ROOT}/tools/cmake/Codegen.cmake)
 
-  if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-  endif()
-
   set(_common_compile_options -Wno-deprecated-declarations -fPIC)
 
   # Find prebuilt libraries. executorch package should contain portable_ops_lib,

backends/cadence/fusion_g3/operators/CMakeLists.txt

@@ -14,10 +14,6 @@ endif()
 include(${EXECUTORCH_ROOT}/tools/cmake/Utils.cmake)
 include(${EXECUTORCH_ROOT}/tools/cmake/Codegen.cmake)
 
-if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-endif()
-
 # ATen compliant ops that are needed to run this model.
 set(_aten_ops__srcs
     "${EXECUTORCH_ROOT}/kernels/portable/cpu/util/activation_ops_util.cpp"

backends/cadence/hifi/operators/CMakeLists.txt

@@ -14,10 +14,6 @@ endif()
 include(${EXECUTORCH_ROOT}/tools/cmake/Utils.cmake)
 include(${EXECUTORCH_ROOT}/tools/cmake/Codegen.cmake)
 
-if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-endif()
-
 # ATen compliant ops that are needed to run this model.
 set(_aten_ops__srcs
     "${EXECUTORCH_ROOT}/backends/cadence/hifi/operators/op_add.cpp"

backends/cadence/reference/operators/CMakeLists.txt

@@ -14,10 +14,6 @@ endif()
 include(${EXECUTORCH_ROOT}/tools/cmake/Utils.cmake)
 include(${EXECUTORCH_ROOT}/tools/cmake/Codegen.cmake)
 
-if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-endif()
-
 # ATen compliant ops that are needed to run this model.
 set(_aten_ops__srcs
     "${CMAKE_CURRENT_SOURCE_DIR}/op_add.cpp"

backends/cortex_m/CMakeLists.txt

@@ -23,10 +23,6 @@ endif()
 include(${EXECUTORCH_ROOT}/tools/cmake/Utils.cmake)
 include(${EXECUTORCH_ROOT}/tools/cmake/Codegen.cmake)
 
-if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-endif()
-
 # Cortex-M ops kernel sources
 set(_cortex_m_kernels__srcs
     ${CMAKE_CURRENT_SOURCE_DIR}/ops/op_quantize_per_tensor.cpp

backends/vulkan/CMakeLists.txt

@@ -24,10 +24,6 @@ if(NOT RUNTIME_PATH)
   set(RUNTIME_PATH ${CMAKE_CURRENT_SOURCE_DIR}/runtime)
 endif()
 
-if(NOT PYTHON_EXECUTABLE)
-  set(PYTHON_EXECUTABLE python3)
-endif()
-
 # Include this file to access target_link_options_shared_lib This is required to
 # provide access to target_link_options_shared_lib which allows libraries to be
 # linked with the --whole-archive flag. This is required for libraries that

backends/vulkan/test/op_tests/CMakeLists.txt

@@ -45,10 +45,6 @@ find_library(LIB_TORCH torch HINTS ${TORCH_INSTALL_PREFIX}/lib)
 find_library(LIB_TORCH_CPU torch_cpu HINTS ${TORCH_INSTALL_PREFIX}/lib)
 find_library(LIB_C10 c10 HINTS ${TORCH_INSTALL_PREFIX}/lib)
 
-if(NOT PYTHON_EXECUTABLE)
-  set(PYTHON_EXECUTABLE python3)
-endif()
-
 # Third party include paths
 
 set(VULKAN_THIRD_PARTY_PATH ${CMAKE_CURRENT_SOURCE_DIR}/../../third-party)

backends/xnnpack/CMakeLists.txt

@@ -25,10 +25,6 @@ endif()
 
 include(${EXECUTORCH_ROOT}/tools/cmake/Utils.cmake)
 
-if(NOT PYTHON_EXECUTABLE)
-  resolve_python_executable()
-endif()
-
 # NB: Enabling this will serialize execution of delegate instances Keeping this
 # OFF by default to maintain existing behavior, to be revisited.
 option(EXECUTORCH_XNNPACK_SHARED_WORKSPACE