clarify the role of the trainer class
clarify the role of the trainer class

wkentaro/chainer,keisuke-umezawa/chainer,chainer/chainer,chainer/chainer,okuta/chainer,okuta/chainer,chainer/chainer,hvy/chainer,wkentaro/chainer,okuta/chainer,hvy/chainer,wkentaro/chainer,niboshi/chainer,pfnet/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,niboshi/chainer,niboshi/chainer,chainer/chainer,tkerola/chainer,hvy/chainer,wkentaro/chainer,niboshi/chainer,okuta/chainer,keisuke-umezawa/chainer,hvy/chainer