check_double_backward to support functions with possible dtype demotion
check_double_backward to support functions with possible dtype demotion

niboshi/chainer,chainer/chainer,pfnet/chainer,hvy/chainer,wkentaro/chainer,wkentaro/chainer,hvy/chainer,chainer/chainer,wkentaro/chainer,okuta/chainer,chainer/chainer,wkentaro/chainer,hvy/chainer,okuta/chainer,hvy/chainer,niboshi/chainer,niboshi/chainer,okuta/chainer,chainer/chainer,okuta/chainer,niboshi/chainer