Give targets that link torch_cpu_library torch's compile settings back

Linking the torch target does more than link. TorchConfig also gives it
TORCH_INCLUDE_DIRS, which CMake treats as system headers because the
target is imported, TORCH_CXX_FLAGS, and on recent torch a minimum C++
standard (C++20). torch_cpu_library, which our targets now link instead
so that a CPU-only torch can load them, only passes on torch/include.

So those targets changed. 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.

find_package_torch now copies torch's interface include directories,
compile options and compile features to torch_cpu_library. It does this
once, also when the project found torch before calling it. Every target
that links torch_cpu_library, directly or through a PUBLIC link such as
custom_ops_aot_lib, now gets these settings, and a new link site cannot
forget them. With a CUDA build of PyTorch, torch also carries the CUDA
include directory and definitions of torch_cuda_library. Those are not
copied, since these targets do not use CUDA. Changing torch_cpu_library
itself also affects any other target in the build that links it. When
there is no torch_cpu_library, for example with a static libtorch or a
parent project that defines its own torch target, nothing is copied.

A new test configures a small project and checks that a target linking
torch_cpu_library, directly or through a PUBLIC link, gets torch's C++
API include directory, TORCH_CXX_FLAGS and, when torch sets one, its C++
standard. It checks both when find_package_torch finds torch and when
the project found it first. A second test checks that
find_package_torch still configures when only a torch target exists.
Both pass with CPU and CUDA builds of torch. The first fails without
this change, and the second fails if the copy does not first check that
torch_cpu_library exists.

Author: shoumikhin

.ci/scripts/tests/test_torch_cpu_library_cmake.py

@@ -0,0 +1,143 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import importlib.util
+import json
+import shlex
+import shutil
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[3]
+
+
+def _compile_flags(entry: dict) -> set:
+    """The flags of one compile command, without the compiler, source and outputs."""
+    flags = set()
+    arguments = iter(shlex.split(entry["command"])[1:])
+    for argument in arguments:
+        if argument in ("-o", "-c", "-MT", "-MF"):
+            next(arguments)
+        elif argument == "-isystem":
+            flags.add(f"-isystem {next(arguments)}")
+        elif argument != "-MD":
+            flags.add(argument)
+    return flags
+
+
+def _copied_from_torch(flag: str) -> bool:
+    """Whether a flag comes from what find_package_torch copies from torch.
+
+    A CUDA build of torch also gives torch the CUDA toolkit include dir and torch_cuda's
+    definitions, which torch_cpu_library correctly does not carry.
+    """
+    return (
+        flag == "-DET_TORCH_CXX_FLAGS_PROBE"
+        or flag.startswith("-std=")
+        or (flag.startswith("-isystem ") and flag.endswith("torch/csrc/api/include"))
+    )
+
+
+@unittest.skipIf(sys.platform == "win32", "Visual Studio writes no compile commands")
+class TestTorchCpuLibraryCMake(unittest.TestCase):
+    def test_torch_cpu_library_gets_torch_settings(self):
+        cmake = shutil.which("cmake")
+        if cmake is None or importlib.util.find_spec("torch") is None:
+            self.skipTest("CMake and torch are required")
+
+        found_first = (
+            "add_torch_to_cmake_prefix_path()\nfind_package(Torch CONFIG REQUIRED)"
+        )
+        for find_torch in ("", found_first):
+            with self.subTest(found_first=bool(find_torch)):
+                self._check(cmake, find_torch)
+
+    def _check(self, cmake: str, find_torch: str) -> None:
+        with tempfile.TemporaryDirectory() as directory:
+            source = Path(directory)
+            build = source / "build"
+            (source / "probe.cpp").write_text("int probe() { return 0; }\n")
+            (source / "CMakeLists.txt").write_text(
+                f"""cmake_minimum_required(VERSION 3.24)
+project(TorchCpuLibraryProbe LANGUAGES CXX)
+include("${{EXECUTORCH_ROOT}}/tools/cmake/Utils.cmake")
+{find_torch}
+find_package_torch()
+add_library(links_torch OBJECT probe.cpp)
+target_link_libraries(links_torch PRIVATE torch)
+add_library(links_torch_cpu_library OBJECT probe.cpp)
+target_link_libraries(links_torch_cpu_library PRIVATE torch_cpu_library)
+add_library(links_it_publicly STATIC probe.cpp)
+target_link_libraries(links_it_publicly PUBLIC torch_cpu_library)
+add_library(consumer OBJECT probe.cpp)
+target_link_libraries(consumer PRIVATE links_it_publicly)
+"""
+            )
+            result = subprocess.run(
+                [
+                    cmake,
+                    "-S",
+                    str(source),
+                    "-B",
+                    str(build),
+                    f"-DEXECUTORCH_ROOT={ROOT}",
+                    f"-DPYTHON_EXECUTABLE={sys.executable}",
+                    "-DCMAKE_EXPORT_COMPILE_COMMANDS=ON",
+                    # Recent torch sets no TORCH_CXX_FLAGS, so set one to check it travels too.
+                    "-DTORCH_CXX_FLAGS=-DET_TORCH_CXX_FLAGS_PROBE",
+                ],
+                capture_output=True,
+                text=True,
+            )
+            self.assertEqual(result.returncode, 0, result.stdout + result.stderr)
+
+            flags = {}
+            for entry in json.loads((build / "compile_commands.json").read_text()):
+                target = Path(entry["output"]).parent.name.removesuffix(".dir")
+                flags[target] = _compile_flags(entry)
+            expected = {
+                flag for flag in flags["links_torch"] if _copied_from_torch(flag)
+            }
+            self.assertIn("-DET_TORCH_CXX_FLAGS_PROBE", expected)
+            self.assertTrue(any(flag.startswith("-isystem ") for flag in expected))
+            for target in ("links_torch_cpu_library", "links_it_publicly", "consumer"):
+                self.assertEqual(expected - flags[target], set(), target)
+
+    def test_find_package_torch_without_torch_cpu_library(self):
+        """A parent project or a static libtorch can define torch alone."""
+        cmake = shutil.which("cmake")
+        if cmake is None:
+            self.skipTest("CMake is required")
+        with tempfile.TemporaryDirectory() as directory:
+            source = Path(directory)
+            (source / "CMakeLists.txt").write_text(
+                """cmake_minimum_required(VERSION 3.24)
+project(TorchWithoutTorchCpuLibrary LANGUAGES NONE)
+include("${EXECUTORCH_ROOT}/tools/cmake/Utils.cmake")
+add_library(torch INTERFACE)
+find_package_torch()
+"""
+            )
+            result = subprocess.run(
+                [
+                    cmake,
+                    "-S",
+                    str(source),
+                    "-B",
+                    str(source / "build"),
+                    f"-DEXECUTORCH_ROOT={ROOT}",
+                ],
+                capture_output=True,
+                text=True,
+            )
+            self.assertEqual(result.returncode, 0, result.stdout + result.stderr)
+
+
+if __name__ == "__main__":
+    unittest.main()

tools/cmake/Utils.cmake

@@ -278,6 +278,36 @@ macro(find_package_torch)
     add_torch_to_cmake_prefix_path()
     find_package(Torch CONFIG REQUIRED)
   endif()
+  # TorchConfig gives its include dirs, compile flags and C++ standard to torch
+  # only. Our targets link torch_cpu_library instead, so copy them there once,
+  # also when the project found torch before calling this. This affects every
+  # target in the build that links torch_cpu_library.
+  if(TARGET torch_cpu_library)
+    get_target_property(
+      _torch_settings_copied torch_cpu_library EXECUTORCH_TORCH_SETTINGS_COPIED
+    )
+    if(NOT _torch_settings_copied)
+      foreach(_torch_property
+              INTERFACE_INCLUDE_DIRECTORIES INTERFACE_COMPILE_OPTIONS
+              INTERFACE_COMPILE_FEATURES
+      )
+        get_target_property(_torch_value torch ${_torch_property})
+        if(_torch_value)
+          set_property(
+            TARGET torch_cpu_library
+            APPEND
+            PROPERTY ${_torch_property} ${_torch_value}
+          )
+        endif()
+      endforeach()
+      set_property(
+        TARGET torch_cpu_library PROPERTY EXECUTORCH_TORCH_SETTINGS_COPIED TRUE
+      )
+    endif()
+    unset(_torch_settings_copied)
+    unset(_torch_property)
+    unset(_torch_value)
+  endif()
 endmacro()
 
 # Modify ${targetName}'s INTERFACE_INCLUDE_DIRECTORIES by wrapping each entry in