fix _div_broadcast_helper in test_operation.py
fix _div_broadcast_helper in test_operation.py

nudles/incubator-singa,apache/incubator-singa,apache/incubator-singa,nudles/incubator-singa,apache/incubator-singa,nudles/incubator-singa,nudles/incubator-singa,apache/incubator-singa,apache/incubator-singa,nudles/incubator-singa