Machine learning and neural networks have never been sexier. The top experts in artificial intelligence are speaking before thousands, signing book deals and attracting gobs of money. The sector is moving from tech niche to center stage stardom. Take for example Kai-Fu Lee, the noted investor, former president of Google China and AI researcher. On Friday, at two speaking engagements in San Francisco, Lee basked in the kind of attention typically reserved for rock stars, not technologists. Young men and women in business attire flocked to him. Investors sought introductions. Even school children asked for autographs.