Talk about impulsive shoppers! As Beyonce was headed to Nobu 57 the other night, she saw the blue dress Hilary Swank wore the night she won the Oscar for “Million Dollar Baby” in the window of the Guy Laroche flagship boutique. The next evening, Beyonce came back to 57th Street and bought it in white without even trying it on. The staff gushed about how sweet, gracious and soft-spoken she was. And they speculated that the dress could easily be worn as a wedding gown if her long-awaited nuptials with Jay-Z ever take place.