remove hstack and vstack fallbacks
remove hstack and vstack fallbacks

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