Benchmark backward execution.

        Note:
            If backward execution throws any exception,
            this benchmark system considers the error is because the function
            doesn't support backward operation, then set the benchmark
            ``None``.

def benchmark_backward(self):
        """Benchmark backward execution.

        Note:
            If backward execution throws any exception,
            this benchmark system considers the error is because the function
            doesn't support backward operation, then set the benchmark
            ``None``.

        """
        try:
            self._benchmark_backward()
        except RuntimeError as e:
            # Seems like not implemented.
            print(e)
            self.mod_ext.synchronize(**self.ext_kwargs)
            self.backward_stat = None