code: def _init_grad(self):
          
          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)
docs: Initialize grad buffers.