Fix update.py for multi optimizers
Fix update.py for multi optimizers

chainer/chainer,niboshi/chainer,delta2323/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,ktnyt/chainer,kiyukuta/chainer,keisuke-umezawa/chainer,hvy/chainer,ktnyt/chainer,jnishi/chainer,ronekko/chainer,niboshi/chainer,ysekky/chainer,jnishi/chainer,wkentaro/chainer,chainer/chainer,wkentaro/chainer,okuta/chainer,ktnyt/chainer,okuta/chainer,chainer/chainer,pfnet/chainer,okuta/chainer,niboshi/chainer,keisuke-umezawa/chainer,chainer/chainer,cupy/cupy,okuta/chainer,hvy/chainer,keisuke-umezawa/chainer,cupy/cupy,hvy/chainer,keisuke-umezawa/chainer,tkerola/chainer,kashif/chainer,jnishi/chainer,aonotas/chainer,ktnyt/chainer,rezoo/chainer,anaruse/chainer,wkentaro/chainer,cupy/cupy,jnishi/chainer,cupy/cupy