code: def grad(self, ctx=None):
          
          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, ctx)
docs: Returns a gradient buffer for this parameter on one context.
  
          Parameters
          ----------
          ctx : Context
              Desired context.