Whatever happened to the Broadway producer, the networking genius who could bring together the right author, the right composer, and the right director to create a great show? These days, most Broadway hits are corporate concoctions, based on Disney movies or the rock bands of yesteryear, and there doesn’t seem to be much room for the old-school masters. A new fellowship program is aiming to train the producers of tomorrow, but no one seems quite certain of what that job will entail.