Make elementwise._reduce_dims efficient
Make elementwise._reduce_dims efficient

minhpqn/chainer,keisuke-umezawa/chainer,wkentaro/chainer,hidenori-t/chainer,cupy/cupy,ysekky/chainer,muupan/chainer,wkentaro/chainer,truongdq/chainer,sinhrks/chainer,hvy/chainer,muupan/chainer,niboshi/chainer,chainer/chainer,sinhrks/chainer,hvy/chainer,t-abe/chainer,ktnyt/chainer,ktnyt/chainer,delta2323/chainer,pfnet/chainer,jnishi/chainer,kikusu/chainer,kiyukuta/chainer,ktnyt/chainer,kashif/chainer,ytoyama/yans_chainer_hackathon,cupy/cupy,tigerneil/chainer,niboshi/chainer,keisuke-umezawa/chainer,tkerola/chainer,chainer/chainer,okuta/chainer,cupy/cupy,niboshi/chainer,yanweifu/chainer,benob/chainer,hvy/chainer,jnishi/chainer,jnishi/chainer,chainer/chainer,rezoo/chainer,masia02/chainer,ktnyt/chainer,wkentaro/chainer,kikusu/chainer,benob/chainer,anaruse/chainer,aonotas/chainer,laysakura/chainer,okuta/chainer,AlpacaDB/chainer,Kaisuke5/chainer,umitanuki/chainer,cemoody/chainer,truongdq/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,cupy/cupy,jnishi/chainer,chainer/chainer,tscohen/chainer,ronekko/chainer,wkentaro/chainer,keisuke-umezawa/chainer,AlpacaDB/chainer,sou81821/chainer,okuta/chainer,t-abe/chainer,niboshi/chainer,ikasumi/chainer,1986ks/chainer