Fix chainerx max_pool double backward gets aborted
Fix chainerx max_pool double backward gets aborted

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