support cleargrad in optimizer hooks
support cleargrad in optimizer hooks

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