fix the type annotation (pytorch#1251)

Summary:
Pull Request resolved: pytorch#1251

for https://www.internalfb.com/diff/D51033222?dst_version_fbid=1068426881261755&transaction_fbid=895472858810060

Reviewed By: stephenyan1231

Differential Revision: D51490247

fbshipit-source-id: 585bf4e954a1f802d4ab04b9bf8c35bb3d9bf5cd

exir/backend/partitioner.py

@@ -7,7 +7,7 @@
 from abc import ABC, abstractmethod
 from dataclasses import dataclass
 from types import MappingProxyType
-from typing import Dict, List, NamedTuple, Union
+from typing import Dict, List, Mapping, NamedTuple, Union
 
 from executorch.exir.backend.backend_details import enforcedmethod
 from executorch.exir.backend.compile_spec_schema import CompileSpec
@@ -56,17 +56,15 @@ class Partitioner(ABC):
 
     def __init__(
         self,
-        spec: MappingProxyType[Union[str, int, float, bool], object] = MappingProxyType(
-            {}
-        ),
+        spec: Mapping[Union[str, int, float, bool], object] = MappingProxyType({}),
     ):
         self._spec = spec
 
     def __call__(self, exported_program: ExportedProgram) -> PartitionResult:
         return self.partition(exported_program)
 
     @property
-    def spec(self) -> MappingProxyType[Union[str, int, float, bool], object]:
+    def spec(self) -> Mapping[Union[str, int, float, bool], object]:
         return self._spec
 
     @enforcedmethod