set lstm lstmp unsed pointer to null

paddle/fluid/operators/lstm_op.h

@@ -311,6 +311,10 @@ class LSTMGradKernel : public framework::OpKernel<T> {
         lstm_grad.prev_state_grad = c0_g ? ordered_c0_g.data<T>() : nullptr;
       }
 
+      // lstm_value.output_value not used in bp, set to null
+      // lstm_grad.state_active_grad not used in bp, set to null
+      lstm_value.output_value = nullptr;
+      lstm_grad.state_active_grad = nullptr;
       int cur_batch_size = bend - bstart;
       math::LstmUnitGradFunctor<DeviceContext, T>::compute(
           device_ctx, lstm_value, lstm_grad, frame_size, cur_batch_size,

paddle/fluid/operators/lstmp_op.h

@@ -405,6 +405,11 @@ class LSTMPGradKernel : public framework::OpKernel<T> {
       }
 
       int cur_batch_size = bend - bstart;
+      // lstm_value.output_value not used in bp, set to null
+      // lstm_grad.state_active_grad not used in bp, set to null
+      lstm_value.output_value = nullptr;
+      lstm_grad.state_active_grad = nullptr;
+
       math::LstmUnitGradFunctor<DeviceContext, T>::compute(
           device_ctx, lstmp_value, lstmp_grad, frame_size, cur_batch_size,
           gate_act, cell_act, cand_act);