Merge pull request chainer#4083 from hvy/release-v4.0.0b2

Update to v4.0.0b2

Merge pull request chainer#4082 from kmaehashi/release-v3.2.0

Update to v3.2.0

Merge pull request chainer#3896 from kmaehashi/v4.0.0b1

update to version v4.0.0b1

Merge pull request chainer#3895 from hvy/v3.1.0

Update to v3.1.0

Merge pull request chainer#3585 from hvy/v4.0.0a1

Update to v4.0.0a1

Merge pull request chainer#3584 from kmaehashi/release-v3.0.0

Update version to v3.0.0

Merge pull request chainer#3373 from beam2d/v3.0.0rc1

Update to v3.0.0rc1

Merge pull request chainer#3374 from gwtnb/update-v2.1.0

Update to v2.1.0

Merge pull request chainer#3083 from mitmul/v3b1

Update version to v3.0.0b1

Merge pull request chainer#3082 from delta2323/v2.0.2

Update to v2.0.2