do nansum only when necessary
do nansum only when necessary

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