A handful of venture capital firms are set to see huge returns from their early bets on Scale AI, following Meta Platforms Inc.’s $14.3 billion mega investment in the artificial intelligence startup. The biggest winner is Accel, one of Silicon Valley’s oldest venture firms, which first backed Scale almost a decade ago, when the startup’s chief executive officer was still a teenager. Accel expects a windfall of more than $2.5 billion, according to a person familiar with the matter who asked not t...