Thieves in California drove off with two Nvidia-branded trailers expecting a fortune in AI chips. When they broke open the doors, they found 40,000 pounds of sand. The trailers belonged to PlusAI, an autonomous trucking startup that partners with Nvidia. To test how a self-driving truck handles at full load, you have to actually load the truck, so PlusAI fills its test trailers with 20,000 pounds each of the cheapest ballast on earth. The truck cabs holding the actual computing hardware were…