Support interface python assignment as an attribute (#26734)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26734
This PR added the python assignment for interface as an attribute in the
module, it enables any object that implicitly inheriting the specific
interface to be able to be assigned to the interface type in python.
Serialization support for interface/class assignment will be done in the
follow up PR
Test Plan: Imported from OSS
Differential Revision: D17742708
Pulled By: wanchaol
fbshipit-source-id: a0a2d8c74b60ed3fa6c05e1b0d49b7ad1abc670b
