force coeff to be cupy.ndarray
force coeff to be cupy.ndarray

Currently, ChainerX implementation seems to fall back to
AveragePoolingNDGrad.forward_gpu even though `coeff` is
chainerx.ndarray and `gy` is cupy.ndarray.

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