code: def list_grad(self):
          
          if self._data is not None and self._grad is None:
              raise RuntimeError(
                  "Cannot get gradient array for Parameter '%s' " \
                  "because grad_req='null'"%(self.name))
          return self._check_and_get(self._grad, list)
docs: Returns gradient buffers on all contexts, in the same order
          as :py:meth:`values`.