LOS ANGELES (Reuters) - The American Music Awards rings in its 40th year on Sunday, with top nominees like Rihanna and Nicki Minaj battling for the top trophies and Stevie Wonder leading a tribute to the show's late founder, Dick Clark. Variety is the key to this year's three-hour ceremony from Los Angeles, with performers including Canadian pop star Justin Bieber, 1990s ska-punk band No Doubt, alt-rockers Linkin Park, country-pop darling Taylor Swift, Korean Internet sensation Psy and British-Irish boyband The Wanted. ...