Nvidia’s blockbuster earnings late Wednesday set up the stock market for a ferocious rebound as the chipmaker appeared to ease fears that an AI bubble was about to pop.Thursday began with a massive rally with the Dow Jones Industrial Average up 700 points, regaining some ground after an earlier AI-related selloff. Upbeat results from retail giant Walmart helped too.But the market suddenly turned lower, and the Dow lost 300 points, leaving Wall Street wondering what the heck happened. Some commentators point ...