Get dictionary representation of gradient arrays.

        Returns
        -------
        grad_dict : dict of str to NDArray
            The dictionary that maps name of arguments to gradient arrays.

def grad_dict(self):
        """Get dictionary representation of gradient arrays.

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