Bret Taylor, chairman of the board of directors of OpenAI, attends the annual Allen and Co. Sun Valley Media and Technology Conference at the Sun Valley Resort in Sun Valley, Idaho, on July 8, 2025.David A. Grogan | CNBCBret Taylor's artificial intelligence startup Sierra has just joined an exclusive club: The company sports a new $10 billion valuation after raising $350 million in fresh capital.Sierra is one of just a handful of AI startups, including OpenAI, Anthropic, xAI, Safe Superintellige...