Added stub cudnn BN functions so that readthedocs should build.
Added stub cudnn BN functions so that readthedocs should build.

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