For the past three years, the head-turning gowns on patrons at New York City Ballet’s fall gala have had major competition — the costumes onstage. Thanks to the efforts of board vice-chair Sarah Jessica Parker, the choreographers who create for these evenings have the opportunity to work with designers from outside the ballet world (often Parker herself helps make connections). This year’s roster of choreographers and designers spoke about making dance and design meet for the fourth annual Fall Fashion gala tonight.