code: def grad_dict(self):
          
          if self._grad_dict is None:
              self._grad_dict = Executor._get_dict(
                  self._symbol.list_arguments(), self.grad_arrays)
          return self._grad_dict
docs: Get dictionary representation of gradient arrays.
  
          Returns
          -------
          grad_dict : dict of str to NDArray
              The dictionary that maps name of arguments to gradient arrays.