Revert "Delete functorch C extension entirely. (#163340)"

This reverts commit 1faf6367e396b1d0894e8735912a47ac465f469d.

Reverted https://github.com/pytorch/pytorch/pull/163340 on behalf of https://github.com/wdvr due to temporary revert to pull out #162659 ([comment](https://github.com/pytorch/pytorch/pull/163340#issuecomment-3317105243))

setup.py

@@ -382,6 +382,12 @@ def _get_package_path(package_name: str) -> Path:
 BUILD_LIBTORCH_WHL = str2bool(os.getenv("BUILD_LIBTORCH_WHL"))
 BUILD_PYTHON_ONLY = str2bool(os.getenv("BUILD_PYTHON_ONLY"))
 
+# set up appropriate env variables
+if BUILD_LIBTORCH_WHL:
+    # Set up environment variables for ONLY building libtorch.so and not libtorch_python.so
+    # functorch is not supported without python
+    os.environ["BUILD_FUNCTORCH"] = "OFF"
+
 if BUILD_PYTHON_ONLY:
     os.environ["BUILD_LIBTORCHLESS"] = "ON"
     os.environ["LIBTORCH_LIB_PATH"] = (_get_package_path("torch") / "lib").as_posix()
@@ -1244,6 +1250,21 @@ class build_ext(setuptools.command.build_ext.build_ext):
     def build_extensions(self) -> None:
         self.create_compile_commands()
 
+        build_lib = Path(self.build_lib).resolve()
+
+        # Copy functorch extension
+        for ext in self.extensions:
+            if ext.name != "functorch._C":
+                continue
+            fullname = self.get_ext_fullname(ext.name)
+            filename = Path(self.get_ext_filename(fullname))
+            src = filename.with_stem("functorch")
+            dst = build_lib / filename
+            if src.exists():
+                report(f"Copying {ext.name} from {src} to {dst}")
+                dst.parent.mkdir(parents=True, exist_ok=True)
+                self.copy_file(src, dst)
+
         super().build_extensions()
 
     def get_outputs(self) -> list[str]:
@@ -1531,6 +1552,11 @@ def configure_extension_build() -> tuple[
     )
     ext_modules.append(C)
 
+    # These extensions are built by cmake and copied manually in build_extensions()
+    # inside the build_ext implementation
+    if cmake_cache_vars["BUILD_FUNCTORCH"]:
+        ext_modules.append(Extension(name="functorch._C", sources=[]))
+
     cmdclass = {
         "bdist_wheel": bdist_wheel,
         "build_ext": build_ext,