Silicon Valley's earliest stage companies are getting a major boost from artificial intelligence.Startup accelerator Y Combinator -- known for backing Airbnb, Dropbox and Stripe -- this week held its annual demo day in San Francisco, where founders pitched their startups to an auditorium of potential venture capital investors.Y Combinator CEO Garry Tan told CNBC that this group is growing significantly faster than past cohorts and with actual revenue. For the last nine months, the entire batch o...