Last Laugh: The fat lady sang on "Cop Rock" Wednesday night, literally, as the full-page ad run by Steven Bochco Productions in Daily Variety promised. Though Bochco's musical police drama was canceled because of horrendous ratings nearly two months ago, the 11th and last episode of the show aired Wednesday. The final scene, the final song, featured characters shaking their heads in disbelief over the show's cancellation and joking "that it really ain't over until the fat lady sings." On cue, a singing fat lady, dressed in purple sequins, descended on a trapeze. Meanwhile, the cast sang their goodbys, including one quip that compared the expensive series to the catastrophic movie failure "Heaven's Gate."