The American Music Awards brought the style last night, with seriously sexy red-carpet dresses from Kendall and Kylie Jenner, Gigi Hadid, and Selena Gomez (her Givenchy dress was backless, guys!). For some of the celebs, the actual ceremony wasn't the only chance to show off their style prowess either: A whole group decamped to Justin Bieber's after-party at L.A. restaurant the Nice Guy, stopping for quick changes along the way to pull on something fresh to party in. Kylie Jenner swapped her…