Add refs for sinc and sgn (#85142)
This PR superseded https://github.com/pytorch/pytorch/pull/80171
This does not add the ref for `special.sinc` as I was getting some
errors. This should be added to https://github.com/pytorch/pytorch/pull/84957
(cc @nkaretnikov)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85142
Approved by: https://github.com/ngimel, https://github.com/mruberry
