Fix to use ChainerX when `reduce=mean` and `normalize=True`
Fix to use ChainerX when `reduce=mean` and `normalize=True`

chainer/chainer,niboshi/chainer,hvy/chainer,chainer/chainer,hvy/chainer,hvy/chainer,pfnet/chainer,niboshi/chainer,chainer/chainer,chainer/chainer,hvy/chainer,niboshi/chainer,niboshi/chainer