Cupy: Support cuDNN 8.1

[kmaehashi]

[takagi]

• [x] Update chainer-test https://github.com/chainer/chainer-test/pull/633
• [x] Update library installer tool https://github.com/cupy/cupy/pull/4636 (backport https://github.com/cupy/cupy/pull/4674)
• [x] Update wheel distribution https://github.com/cupy/cupy-release-tools/pull/65
• [x] Update document https://github.com/cupy/cupy/pull/4643 (backport https://github.com/cupy/cupy/pull/4700)

A new RNN algo is introduced but I don't touch it here because the enum values of RNN algos are changed between cuDNN v8.0.5 and v8.1 so a naive change breaks the compatibility.

[takagi]

After #4700 is merged, we can close this issue.

[takagi]

I close this issue as the PRs above are all merged.