DC has a major dance festival on the horizon: Velocity DC, the brainchild of Dance MetroDC, the Washington Performing Arts Society, The Harman Center for the Arts, the Washington Ballet and CityDance.   Looking ahead to its premiere on October 2 and 3 the founders staged a launch event, chronicled here, to raise a little money, find common space, have some fun and get the partnerships critical to its success off to the right start.