Add documentation to ``Trainer.is_before_training``
Add documentation to ``Trainer.is_before_training``

hvy/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,chainer/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,wkentaro/chainer,chainer/chainer,chainer/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,pfnet/chainer