make _get_ctrl_spec a class method

Author: anurudhp

qualtran/_infra/gate_with_registers.py

@@ -371,8 +371,9 @@ def __pow__(self, power: int) -> 'GateWithRegisters':
             return Power(bloq, abs(power))
         raise NotImplementedError(f"{self} does not implemented __pow__ for {power=}.")
 
+    @classmethod
     def _get_ctrl_spec(
-        self,
+        cls,
         num_controls: Union[Optional[int], 'CtrlSpec'] = None,
         control_values=None,
         control_qid_shape: Optional[Tuple[int, ...]] = None,