详解见网址 ：https://dato.com/products/create/docs/generated/graphlab.recommender.item_similarity_recommender.ItemSimilarityRecommender.html?__hstc=8457130.bb311fd08c8f8aff1469489de1cfa928.1429598917018.1429604555369.1429606046033.3&__hssc=8457130.3.1429606046033&__hsfp=4211374419#graphlab.recommender.item_similarity_recommender.ItemSimilarityRecommender
