Jensen Huang's departure sent Nvidia into a tailspin, but retail investors saw a buying opportunity. In the first half hour of Thursday, they bought shares for a net $111 million, more than ever before at an opening. The figures come from the research firm Vanda Research, according to Bloomberg. The purchases during the opening were more than 40 times larger than on a normal trading day. Nvidia was already at the center of retail investors' atte…