change to find_method of lite_interpreter API to return nullptr if method not found (#38503)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38503
Modify find_method to not error out if method doesn't
exist to be more similar to:
https://github.com/pytorch/pytorch/blob/master/torch/csrc/jit/api/object.h#L100
Test Plan: run functions as part of mobile ASR
Reviewed By: iseeyuan
Differential Revision: D21466638
fbshipit-source-id: 635bff32539f7495f68dd3b203aaeb108f6283da
