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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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