code: def arg_dict(self):
          
          if self._arg_dict is None:
              self._arg_dict = Executor._get_dict(
                  self._symbol.list_arguments(), self.arg_arrays)
          return self._arg_dict
docs: Get dictionary representation of argument arrrays.
  
          Returns
          -------
          arg_dict : dict of str to NDArray
              The dictionary that maps the names of arguments to NDArrays.
  
          Raises
          ------
          ValueError : if there are duplicated names in the arguments.