Refactor TOSA backend preprocess to use visitor pattern (#1099)

Summary:
No lowering logic change. Only a very large refactor patch to make the current code more modular for easier future maintenance and development.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1099

Reviewed By: shoumikhin, cbilgin

Differential Revision: D50915203

Pulled By: digantdesai

fbshipit-source-id: bcb05fd95ce1cb4b1872a121ac7a88bc81bc4a3b