Add getMethod to PytorchPredictorContainer (#61052)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61052
Implement getMethod in the container in a similar way to getPredictor,
using either Deploy or Script functionality depending on how the container
was initialized and how the gflag deploy override are set.
Test Plan: Add new unit test
Reviewed By: houseroad
Differential Revision: D29346969
fbshipit-source-id: 08e95ee96d533f5a7cc9c8f9b1c53751715c9181
