Test AveragePool backward and double backward
Test AveragePool backward and double backward

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