SINGA-28 Fix a bug from toplogy sort of Graph
SINGA-28 Fix a bug from toplogy sort of Graph

There is a bug in the Sort function of graph.cc,
which would push one node into the 'tmp' queue for
multiple times if this node has multiple source nodes.

Solution: use an unordered_set to store all nodes in the queue;
push a node only when it is not in the 'tmp' queue.

ijingo/incubator-singa,apache/incubator-singa,apache/incubator-singa,nusdbsystem/incubator-singa,nudles/incubator-singa,pthaike/incubator-singa,venkatsatish/incubator-singa,nudles/incubator-singa,apache/incubator-singa,devfreesg/incubator-singa,kaiping/incubator-singa,nusdbsystem/incubator-singa,jaejaejae/incubator-singa,SeSaMe-NUS/incubator-singa,jinyangturbo/incubator-singa,devfreesg/incubator-singa,wangsheng1001/incubator-singa,nusdbsystem/incubator-singa,xindaya/incubator-singa,devfreesg/incubator-singa,nudles/incubator-singa,cac2003/incubator-singa,wangsheng1001/incubator-singa,pthaike/incubator-singa,venkatsatish/incubator-singa,cac2003/incubator-singa,nusdbsystem/incubator-singa,cszqwe/incubator-singa,ijingo/incubator-singa,apache/incubator-singa,wangsheng1001/incubator-singa,cac2003/incubator-singa,jaejaejae/incubator-singa,zhoujingbo/incubator-singa,litoupu/incubator-singa,nudles/incubator-singa,litoupu/incubator-singa,jinyangturbo/incubator-singa,kaiping/incubator-singa,pthaike/incubator-singa,litoupu/incubator-singa,cszqwe/incubator-singa,devfreesg/incubator-singa,SeSaMe-NUS/incubator-singa,apache/incubator-singa,litoupu/incubator-singa,zhoujingbo/incubator-singa,venkatsatish/incubator-singa,cszqwe/incubator-singa,ijingo/incubator-singa,zhoujingbo/incubator-singa,ijingo/incubator-singa,xindaya/incubator-singa,xindaya/incubator-singa,kaiping/incubator-singa,kaiping/incubator-singa,wangsheng1001/incubator-singa,nudles/incubator-singa,ijingo/incubator-singa,SeSaMe-NUS/incubator-singa,cac2003/incubator-singa,xindaya/incubator-singa,nusdbsystem/incubator-singa,venkatsatish/incubator-singa,kaiping/incubator-singa,zhoujingbo/incubator-singa,jinyangturbo/incubator-singa,wangsheng1001/incubator-singa,cszqwe/incubator-singa,SeSaMe-NUS/incubator-singa,jinyangturbo/incubator-singa,pthaike/incubator-singa,jaejaejae/incubator-singa,jaejaejae/incubator-singa