Fix bugs in softmax.py
Fix bugs in softmax.py

ronekko/chainer,ktnyt/chainer,chainer/chainer,tkerola/chainer,hvy/chainer,aonotas/chainer,wkentaro/chainer,chainer/chainer,jnishi/chainer,hvy/chainer,jnishi/chainer,wkentaro/chainer,wkentaro/chainer,kiyukuta/chainer,ysekky/chainer,anaruse/chainer,okuta/chainer,niboshi/chainer,pfnet/chainer,niboshi/chainer,niboshi/chainer,chainer/chainer,keisuke-umezawa/chainer,wkentaro/chainer,keisuke-umezawa/chainer,chainer/chainer,ktnyt/chainer,niboshi/chainer,jnishi/chainer,keisuke-umezawa/chainer,hvy/chainer,jnishi/chainer,keisuke-umezawa/chainer,rezoo/chainer,ktnyt/chainer,okuta/chainer,delta2323/chainer,ktnyt/chainer,okuta/chainer,okuta/chainer,hvy/chainer,kashif/chainer