In the beginning was Cary Grant in ''Bringing Up Baby.'' And he begat Ryan O'Neal in ''What's Up, Doc?'' And he has begotten John Ritter in They All Laughed, another new comedy from Peter Bogdanovich, the former golden boy of Hollywood who has fallen once again into cloning the classics. ''They All Laughed'' is about a pair of private detectives who fall in love with the women they're supposed to be surveilling. It has some humorous moments, especially when Ritter is bumbling around the screen in his archetypically absent-minded role. But the movie never gets one millimeter beneath the surface of its shallow plot or its shallow characters, who plunk their shallow personalities into shallow romances with a regularity that's as dull as it is calculated. The impressive presences of Audrey Hepburn and Ben Gazzara help just a little. Otherwise, it's a hollow and very minor affair.