Expose run_async function on torch::jit::Method (#48607)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48607
This change builds on top of
https://github.com/pytorch/pytorch/pull/46865
further exposing the async interface to `torch::jit::Method`.
added unit test for new `run_async`
Test Plan: `buck test caffe2/test/cpp/jit/...`
Reviewed By: dzhulgakov
Differential Revision: D25219726
fbshipit-source-id: 89743c82a0baa1affe0254c1e2dbf873de8e5c76
