Fix Power gradient dtype
Fix Power gradient dtype

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