Add multi-gpu unit tests on accumulate_grads
Add multi-gpu unit tests on accumulate_grads

chainer/chainer,t-abe/chainer,kashif/chainer,cemoody/chainer,muupan/chainer,okuta/chainer,minhpqn/chainer,tscohen/chainer,ktnyt/chainer,ysekky/chainer,aonotas/chainer,pfnet/chainer,wkentaro/chainer,jnishi/chainer,chainer/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,ktnyt/chainer,jnishi/chainer,AlpacaDB/chainer,benob/chainer,cupy/cupy,jnishi/chainer,niboshi/chainer,niboshi/chainer,sinhrks/chainer,sinhrks/chainer,hvy/chainer,chainer/chainer,muupan/chainer,niboshi/chainer,wkentaro/chainer,wkentaro/chainer,hvy/chainer,wkentaro/chainer,keisuke-umezawa/chainer,t-abe/chainer,truongdq/chainer,chainer/chainer,kiyukuta/chainer,kikusu/chainer,tkerola/chainer,ronekko/chainer,hvy/chainer,okuta/chainer,ktnyt/chainer,cupy/cupy,jnishi/chainer,cupy/cupy,hvy/chainer,rezoo/chainer,kikusu/chainer,okuta/chainer,ktnyt/chainer,anaruse/chainer,benob/chainer,truongdq/chainer,niboshi/chainer,delta2323/chainer,okuta/chainer,keisuke-umezawa/chainer,cupy/cupy,AlpacaDB/chainer