Fix ThreadParameterUpdater

The reference return type causes ThreadParameterUpdater.cpp:123 seg fault under gcc5.4.

Change-Id: I7a1c155892722076a7cb48793b83d5ee525747d1

paddle/trainer/ThreadParameterUpdater.h

@@ -79,7 +79,7 @@ protected:
   // The update function for after update operations, such as averager.
   void threadTraverse(const ParameterOptimizer::TraverseCallback& callback,
                       int tid, size_t numThreads, Parameter* para);
-  typedef std::function<const ParameterOptimizer::TraverseCallback&(Parameter*)>
+  typedef std::function<const ParameterOptimizer::TraverseCallback(Parameter*)>
     GetTraverseCallback;
   void traverse(GetTraverseCallback getTraverseCallback);
 };