(Reuters) -U.S. main stock index futures were flat on Tuesday, as investors assessed mixed corporate results from legacy names such as United Parcel Service and General Motors while awaiting a key jobs report that could offer clues into the labor market health. United Parcel Service fell 7% in premarket trading after it forecast annual revenue below Street estimates, as the world's biggest package delivery company faces sluggish demand from e-commerce firms. General Motors, on the other hand, jumped 8.3% after the automotive giant forecast upbeat earnings for 2024 and signaled more capi...