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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 _init_grad(self):
 """Initialize grad buffers."""

        if self.grad_req == 'null':
            self._grad = None
            return

        self._grad = [ndarray.zeros(shape=i.shape, dtype=i.dtype, ctx=i.context,
                                    stype=self._grad_stype) for i in self._data]

        autograd.mark_variables(self._check_and_get(self._data, list),
                                self._grad, self.grad_req)