Robert Pattinson partied with fellow British thespian Sienna Miller before her HBO movie “The Girl” premiered at the Hamptons Film Festival on Saturday. Pattinson, who’s rumored to be getting back with cheating girlfriend Kristen Stewart, was spotted with Miller and two other friends at the Monkey Bar on Friday. Sources said the “Twilight” hunk is a longtime friend of new mom Miller. The group later headed to the Electric Room and hung with club owner Nur Khan.