Reduce the threads number in the LSTM backward kernel to fix the error occurred in GPU GTX 1080.

paddle/operators/math/detail/lstm_gpu_kernel.h

@@ -227,7 +227,7 @@ void gpu_lstm_backward(const platform::DeviceContext& context, Op op,
     grid = dim3(frameBlocks, 1);
   } else {
     /* framePerBlock = 32 batchPerBlock = 32 */
-    threads = dim3(32, 32);
+    threads = dim3(32, 16);
     grid = dim3((frameSize + 32 - 1) / 32, (batchSize + 32 - 1) / 32);
   }
 
@@ -244,6 +244,11 @@ void gpu_lstm_backward(const platform::DeviceContext& context, Op op,
         op, value, grad, frameSize, batchSize, active_node, active_gate,
         active_state);
   }
+
+  cudaStreamSynchronize(stream);
+  // TODO(qingqing): Add cuda error check for each kernel.
+  cudaError_t err = cudaGetLastError();
+  PADDLE_ENFORCE_EQ(err, cudaGetErrorString(err));
 }
 
 }  // namespace detail