Compute negative_sampling in fp32 for fp16 inputs
Compute negative_sampling in fp32 for fp16 inputs

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