Fix the uninitialized gru_value.output_value. (#17197)

test=develop

paddle/fluid/operators/gru_op.h

@@ -144,7 +144,7 @@ class GRUGradKernel : public framework::OpKernel<T> {
         Tensor hidden_prev_grad_t = batch_hidden_grad.Slice(bstart_pre, bstart);
         gru_grad.prev_out_grad = hidden_prev_grad_t.data<T>();
       }
-
+      gru_value.output_value = nullptr;
       math::GRUUnitGradFunctor<DeviceContext, T>::compute(
           dev_ctx, gru_value, gru_grad, frame_size, cur_batch_size, active_node,
           active_gate, origin_mode);