Update chainer/functions/loss/mean_squared_error.py
Update chainer/functions/loss/mean_squared_error.py

Co-Authored-By: hsezhiyan <a0948f4cba8d82571e7c4a487067cae3deb7aa68@users.noreply.github.com>
keisuke-umezawa/chainer,hvy/chainer,wkentaro/chainer,tkerola/chainer,niboshi/chainer,chainer/chainer,chainer/chainer,niboshi/chainer,hvy/chainer,hvy/chainer,wkentaro/chainer,keisuke-umezawa/chainer,okuta/chainer,chainer/chainer,pfnet/chainer,okuta/chainer,keisuke-umezawa/chainer,okuta/chainer,chainer/chainer,keisuke-umezawa/chainer,niboshi/chainer,wkentaro/chainer,niboshi/chainer,hvy/chainer,okuta/chainer,wkentaro/chainer