THE BIG YAWN: NETWORKS IN RATINGS FREE-FALL AT CONVENTION, OPENING NIGHT: ABCNEWS JENNINGS WITH 3.5 RATING/5 SHARE [DOWN FROM 4.5/8 IN 2000]; NBCNEWS BROKAW 3.3/5 [2000:4.8/9]; CBS DAN RATHER 3.2/5 [2000:3.8/7... TRAIL ALL OTHER PRIME-TIME PROGRAMMING [CBS:MIAMI RERUN ON CBS PULLS 8.6 RATING/13 SHARE]... DEVELOPING...