Update chainer/optimizers/adam.py
Update chainer/optimizers/adam.py

Co-Authored-By: crcrpar <5d40c5c70d3d1edb83028cb0acf367584fd843d9@gmail.com>
pfnet/chainer,okuta/chainer,hvy/chainer,keisuke-umezawa/chainer,chainer/chainer,okuta/chainer,chainer/chainer,keisuke-umezawa/chainer,niboshi/chainer,wkentaro/chainer,chainer/chainer,niboshi/chainer,niboshi/chainer,hvy/chainer,chainer/chainer,hvy/chainer,wkentaro/chainer,wkentaro/chainer,wkentaro/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,okuta/chainer,hvy/chainer,tkerola/chainer,niboshi/chainer