Fix ChainerX optimzer fallback for non-default devices
Fix ChainerX optimzer fallback for non-default devices

keisuke-umezawa/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,tkerola/chainer,okuta/chainer,hvy/chainer,pfnet/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,keisuke-umezawa/chainer,chainer/chainer,okuta/chainer,okuta/chainer,chainer/chainer,hvy/chainer,keisuke-umezawa/chainer,niboshi/chainer,hvy/chainer,wkentaro/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,wkentaro/chainer