Fix pyre in arm_backend.py (#7069)

Summary: Pull Request resolved: #7069

Differential Revision: D66475070

Author: kirklandsign

backends/arm/arm_backend.py

@@ -135,7 +135,9 @@ def set_quantize_io(self, quantize_io: bool = False) -> "ArmCompileSpecBuilder":
         self.quantize_io = quantize_io
         return self
 
-    def set_input_order(self, input_order: str = None) -> "ArmCompileSpecBuilder":
+    def set_input_order(
+        self, input_order: Optional[str] = None
+    ) -> "ArmCompileSpecBuilder":
         """
         Reorder the inputs coming in. This may be required when inputs > 1.
         And while using the U55/U85 CompileSpec.