add a warning for script classes (#31069)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31069
Just to clarify that they are still experimental.
Test Plan: Imported from OSS
Differential Revision: D18920496
Pulled By: suo
fbshipit-source-id: d2f3014592a01a21f7fc60a4ce46dd0bfe5e19e9
