To remedy that, some retailers are training their salespeople on when to approach shoppers ??? and when not to.For its part, Wal-Mart is putting checkout in the hands of the shoppers themselves.The retailer is testing its ???Scan & Go??? app, which can be used on Apple devices such as iPads, in more than 200 of its more than 4,000 stores nationwide.The app, which is aimed at reducing long checkout