Robert Pietranton probably would have been aghast but his many friends and colleagues were heartened to gather on Saturday at the Steven J. Ross Theater on the Warner Bros. lot to pay tribute to the long-serving Warner Bros. Television Group PR executive who died in November at the age of 56. Pietranton, as numerous speakers noted, was a very private person who preferred to put the spotlight on others. But his great qualities — incredible work ethic, dedication to the studio, his kindness and