custom class method holder should hold a unique_ptr (#35218)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35218
We should express the ownership semantics directly here. Using
`shared_ptr` makes it too easy to leak ownership by inadvertently
storing a copy.
Test Plan: Imported from OSS
Differential Revision: D20682673
Pulled By: suo
fbshipit-source-id: 32002ee515eb8bb7b37e6d0aac3c0695df4eec79
