Add signature of chainerx.softmax_cross_entropy
Add signature of chainerx.softmax_cross_entropy

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