Thinking Machines Lab, an artificial intelligence startup founded by former OpenAI executive Mira Murati, is in early talks to raise a new round of funding at a roughly $50 billion valuation, according to people familiar with the matter. The new financing, which has not been previously reported, would more than quadruple the startup’s valuation from July and catapult Thinking Machines into the upper ranks of the most valuable private companies less than a year after it launched....