Allow ufunc OpInfos to have no reference (#82348)
The `ref` property was moved down from `{Unary,Binary}UfuncInfo` into
`OpInfo` quite some time ago, but `OpInfo` uses `None` to signal no
reference is available while the others use `_NOTHING`. This makes
everything consistently use `None`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82348
Approved by: https://github.com/ngimel
