Return dummy context by default in Device.create_context()
Return dummy context by default in Device.create_context()

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