By Aditya Soni and Kanchana Chakravarty Dec 11 (Reuters) - Oracle shares sank 13% on Thursday, sparking a tech selloff as the company's massive spending and weak forecasts fanned doubts over how quickly the big bets on AI will pay off. The disappointing predictions ​from the crucial OpenAI cloud-computing partner show the uneven returns from the nascent technology that many business leaders believe is the ‌future but has so far yielded limited productivity gains. Oracle, long a smaller cloud player, ha ...