Add interface declarations to JIT (#25258)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25258
this is the first commit in a series to add interfaces to JIT.
Interfaces allow the specification through a blank python class of an
abstract interface that can be used in type annotations for Script functions.
If a TorchScript class implements all the methods in the interface with
the appropriate types, then it is implicitly considered to implement
that interface.
Follows required:
* implementation of serialization
* implementation in the parser frontend
* better error reporting for explaining why a class does not meet an
interface specification.
Test Plan: Imported from OSS
Differential Revision: D17079963
Pulled By: zdevito
fbshipit-source-id: a9986eeba2d4fdedd0064ce7d459c0251480a5a0
