Made optimizer argument optional
Made optimizer argument optional

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