add comment in code about restrictions on supported dtypes
add comment in code about restrictions on supported dtypes

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