missing _fused_adopt

[winglian]

in this line https://github.com/iShohei220/adopt/blob/main/adopt.py#L486
there seems to be an undefined variable _fused_adopt that doesn't exist anywhere

[iShohei220]

Thank you for pointing it out.
We've fixed it in the latest commit.

Note that we are not currently supporting the fused implementation, so setting fused=True leads to RuntimeError.