Implement torch.nextafter (#42580)
Summary:
Related to https://github.com/pytorch/pytorch/issues/38349.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42580
Reviewed By: smessmer
Differential Revision: D23012260
Pulled By: mruberry
fbshipit-source-id: ce82a63c4ad407ec6ffea795f575ca7c58cd6137
