Add gradient_check.check_double_backward in reference
Add gradient_check.check_double_backward in reference

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