Revert GPU impl
Revert GPU impl

jnishi/chainer,chainer/chainer,ktnyt/chainer,wkentaro/chainer,wkentaro/chainer,okuta/chainer,okuta/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer,niboshi/chainer,niboshi/chainer,aonotas/chainer,keisuke-umezawa/chainer,hvy/chainer,hvy/chainer,keisuke-umezawa/chainer,wkentaro/chainer,pfnet/chainer,jnishi/chainer,jnishi/chainer,tkerola/chainer,jnishi/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,keisuke-umezawa/chainer,ktnyt/chainer,ronekko/chainer,hvy/chainer,okuta/chainer,niboshi/chainer,ktnyt/chainer,chainer/chainer,rezoo/chainer,niboshi/chainer,anaruse/chainer,ktnyt/chainer