Minor fix.

Author: zxybazh

python/tvm/meta_schedule/cost_model/cost_model.py

@@ -35,7 +35,7 @@
 class CostModel(Object):
     """Cost model."""
 
-    def load(self, path: str) -> bool:
+    def load(self, path: str) -> None:
         """Load the cost model from given file location.
 
         Parameters
@@ -45,7 +45,7 @@ def load(self, path: str) -> bool:
         """
         _ffi_api.CostModelLoad(self, path)  # type: ignore # pylint: disable=no-member
 
-    def save(self, path: str) -> bool:
+    def save(self, path: str) -> None:
         """Save the cost model to given file location.
 
         Parameters