[DevX] Enable editable mode (pytorch#7443)

Enable `pip install -e .` with local build flatc and custom ops AOT lib
disabled.

setup.py

@@ -216,8 +216,13 @@ def src_path(self, installer: "InstallerBuildExt") -> Path:
                 file.
         """
         # Share the cmake-out location with CustomBuild.
-        cmake_cache_dir = Path(installer.get_finalized_command("build").cmake_cache_dir)
-
+        build_cmd = installer.get_finalized_command("build")
+        if hasattr(build_cmd, "cmake_cache_dir"):
+            cmake_cache_dir = Path(build_cmd.cmake_cache_dir)
+        else:
+            # If we're in editable mode, use a default or fallback value for cmake_cache_dir
+            # This could be a hardcoded path, or a path derived from the current working directory
+            cmake_cache_dir = Path(".")
         cfg = get_build_type(installer.debug)
 
         if os.name == "nt":
@@ -232,7 +237,14 @@ def src_path(self, installer: "InstallerBuildExt") -> Path:
         srcs = tuple(cmake_cache_dir.glob(self.src))
         if len(srcs) != 1:
             raise ValueError(
-                f"Expected exactly one file matching '{self.src}'; found {repr(srcs)}"
+                f"""Expected exactly one file matching '{self.src}'; found {repr(srcs)}. 
+
+If that file is a CMake-built extension module file, and we are installing in editable mode, please disable the corresponding build option since it's not supported yet.
+
+Try: 
+
+EXECUTORCH_BUILD_FLATC=OFF EXECUTORCH_BUILD_KERNELS_CUSTOM_AOT=OFF pip install -e .
+"""
             )
         return srcs[0]
 
@@ -401,7 +413,11 @@ def run(self):
         # package, and will look like `pip-out/lib`. It can contain multiple
         # python packages, so be sure to copy the files into the `executorch`
         # package subdirectory.
-        dst_root = os.path.join(self.build_lib, self.get_package_dir("executorch"))
+        if self.editable_mode:
+            # In editable mode, the package directory is the original source directory
+            dst_root = self.get_package_dir(".")
+        else:
+            dst_root = os.path.join(self.build_lib, self.get_package_dir("executorch"))
 
         # Create the version file.
         Version.write_to_python_file(os.path.join(dst_root, "version.py"))