Add IMethod interface
Summary:
Expose IMethod interface, which provides a unified interface to either script or python methods backed by torchscript or torchdeploy.
IMethod provides a way to depend on a torch method without depending on a particular runtime implementation such as torchscript or python/deploy.
Test Plan: add unit tests.
Reviewed By: suo
Differential Revision: D29463455
fbshipit-source-id: 903391d9af9fbdd8fcdb096c1a136ec6ac153b7c
