Fork FunctionRef for pytree (#554)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/554

Now there are two FunctionRef classes,
(A) torch::executor::pytree::FunctionRef in executorch/extension/pytree/function_ref.h
(B) torch::executor::FunctionRef in executor/runtime/core/function_ref.h

Rationale: We are aiming for version (B), compared to (A), to have features going forward, and also planning to eventually delete it completely.

Reviewed By: dbort

Differential Revision: D49780124

fbshipit-source-id: 48d27757b0e33fd47287fefef0e622595b9e0c09