fix lstmp bug; test=develop

paddle/fluid/operators/lstmp_op.h

@@ -405,10 +405,10 @@ 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;
+      // lstmp_value.output_value not used in bp, set to null
+      // lstmp_grad.state_active_grad not used in bp, set to null
+      lstmp_value.output_value = nullptr;
+      lstmp_grad.state_active_grad = nullptr;
 
       math::LstmUnitGradFunctor<DeviceContext, T>::compute(
           device_ctx, lstmp_value, lstmp_grad, frame_size, cur_batch_size,