Optimize backward of lo2 and log10
Optimize backward of lo2 and log10

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