torchao setup.py with cmake

Summary:
Initial draft of using cmake in torchao's build process.

Install torchao with:

```
USE_CPP=1 pip install .
```

If on an arm64 machine, it builds the dynamic library for torchao at site-packages/torchao/libtorchao_ops_aten.dylib.  On import of torchao, if this library is found it is loaded.

Differential Revision: D67777662

Author: metascroy

setup.py

@@ -8,7 +8,7 @@
 from datetime import datetime
 import subprocess
 
-from setuptools import find_packages, setup
+from setuptools import find_packages, setup, Extension
 
 current_date = datetime.now().strftime("%Y%m%d")
 
@@ -35,6 +35,12 @@ def read_version(file_path="version.txt"):
 
 use_cpp = os.getenv('USE_CPP')
 
+import platform
+build_torchao_experimental = (
+    use_cpp == "1" and
+    platform.machine().startswith("arm64")
+)
+
 version_prefix = read_version()
 # Version is version.dev year month date if using nightlies and version if not
 version = f"{version_prefix}.dev{current_date}" if os.environ.get("TORCHAO_NIGHTLY") else version_prefix
@@ -50,6 +56,44 @@ def read_version(file_path="version.txt"):
 )
 
 
+# BuildExtension is a subclass of from setuptools.command.build_ext.build_ext
+class TorchAOBuildExt(BuildExtension):
+    def build_extensions(self):
+        cmake_extensions = [ext for ext in self.extensions if isinstance(ext, CMakeExtension)]
+        other_extensions = [ext for ext in self.extensions if not isinstance(ext, CMakeExtension)]
+
+        for ext in cmake_extensions:
+            self.build_cmake(ext)
+        for ext in other_extensions:
+            self.build_other(ext)
+
+    def build_cmake(self, ext):
+        extdir = os.path.abspath(os.path.dirname(self.get_ext_fullpath(ext.name)))
+
+        from distutils.sysconfig import get_python_lib
+        torch_dir = get_python_lib() + "/torch/share/cmake/Torch"
+
+        if not os.path.exists(self.build_temp):
+            os.makedirs(self.build_temp)
+
+        subprocess.check_call(
+            ['cmake', ext.sourcedir, '-DCMAKE_BUILD_TYPE=Release', '-DTORCHAO_BUILD_EXECUTORCH_OPS=OFF', '-DTorch_DIR=' + torch_dir, '-DCMAKE_LIBRARY_OUTPUT_DIRECTORY=' + extdir],
+            cwd=self.build_temp
+        )
+        subprocess.check_call(
+            ['cmake', '--build', '.'],
+            cwd=self.build_temp
+        )
+
+    def build_other(self, ext):
+        super().build_extension(ext)
+
+class CMakeExtension(Extension):
+    def __init__(self, name, sourcedir=''):
+        Extension.__init__(self, name, sources=[])
+        self.sourcedir = os.path.abspath(sourcedir)
+
+
 def get_extensions():
     debug_mode = os.getenv('DEBUG', '0') == '1'
     if debug_mode:
@@ -103,18 +147,25 @@ def get_extensions():
     if use_cuda:
         sources += cuda_sources
 
-    if len(sources) == 0:
-        return None
-
-    ext_modules = [
+    ext_modules = []
+    if len(sources) > 0:
+        ext_modules.append(
         extension(
             "torchao._C",
             sources,
             py_limited_api=True,
             extra_compile_args=extra_compile_args,
             extra_link_args=extra_link_args,
         )
-    ]
+        )
+
+    if build_torchao_experimental:
+        ext_modules.append(
+            CMakeExtension(
+                "torchao.experimental",
+                sourcedir="torchao/experimental",
+            )
+        )
 
     return ext_modules
 
@@ -133,7 +184,7 @@ def get_extensions():
     long_description=open("README.md").read(),
     long_description_content_type="text/markdown",
     url="https://github.com/pytorch/ao",
-    cmdclass={"build_ext": BuildExtension},
+    cmdclass={"build_ext": TorchAOBuildExt},
     options={"bdist_wheel": {
         "py_limited_api": "cp39"
     }},

torchao/__init__.py

@@ -27,6 +27,11 @@
         assert len(so_files) == 1, f"Expected one _C*.so file, found {len(so_files)}"
         torch.ops.load_library(so_files[0])
         from . import ops
+
+        experimental_lib = list(Path(__file__).parent.glob("libtorchao_ops_aten.*"))
+        if len(experimental_lib) > 0:
+            assert len(so_files) == 1, f"Expected at most one libtorchao_ops_aten.* file, found {len(experimental_lib)}"
+            torch.ops.load_library(experimental_lib[0])
     except:
         logging.debug("Skipping import of cpp extensions")