The Ernst Lubitsch retrospective about to unfold at Film Forum will offer a most welcome occasion to gauge the dimensions of the world he celebrated, and sample Lubitsch’s very particular pleasures. He offers a parallel domain of buoyant elegance, a theater of free-floating desire and inextinguishable humor ingeniously stitched together out of the fabric of Austrian operettas and French farces and the plot devices of a hundred forgotten Hungarian plays, flavored by delicate irony and risqué innuendo, where sex is everywhere but just out of sight.