Some 500 members of the Hollywood elite packed into the Directors Guild theater in Los Angeles yesterday to remember leading man Walter Matthau, who died of heart failure on July 1. The crowd included Sophia Loren, Carol Burnett, Diane Keaton, Larry King, Lauren Bacall, Gregory Peck and Angie Dickinson. Like any good Matthau movie, the event was packed with laughs, in spite of itself. Even Matthau's longtime friend and frequent costar Jack Lemmon, though he unsuccessfully fought back tears, elicited a guffaw when he related a story. This one had to do with Matthau's once accompanying him to a sneak preview of a Lemmon movie that was so awful that most of the audience had walked out before the film had finished. When it was over, Lemmon said he asked his buddy what he thought of the film. Matthau's response? "Get out of it!"