adding GRU base function
adding GRU base function

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