Like Lord Byron, LEONARD BERN STEIN awoke one morning to find himself famous. At 25 he substituted for Bruno Walter and conducted the New York Philharmonic entirely without rehearsal. Bernstein sees himself as a composer who conducts, and wants ' to keep on trying to be, in the full sense of that wonderful, word, a musician ... ' He has written everything from song-cycles to symphonies, but is probably still best known as the creator of West Side Story.