guard against inheritance on torchscript classes (#28407)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28407
Given that we do not have support for inheitance or any polymorphism
strategy yet, we should guard against user from using it until we get
the full support so that user won't confuse by the weird behaviors.
Test Plan: Imported from OSS
Differential Revision: D18284310
fbshipit-source-id: f55a224f4190d57926d91ed98f6168d787387eb8
