fix @property (#28395)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28395
Currently property methods are broken in TorchScript because we
basically treat it as an attribute in the existing path: we'll evaluate
the method once and store that as the value forever.
Since lack of property support is easily worked around (just make it
a method), I've opted to just explicitly error to avoid confusion. If
people want it, they can file an issue and we can look at their use
case.
This also helps us nicely clean up some parts of the ScriptModule conversion
path.
Test Plan: Imported from OSS
Reviewed By: shannonzhu
Differential Revision: D18054946
Pulled By: suo
fbshipit-source-id: 7e927836ae687cd2f13a94b9f0af399437fae422
