Add lazy Node base and associated infra (#66601)
Summary:
- Adds Node base class and unit tests
- Also adds metadata utils to enable source code annotation and scope tracking
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66601
Test Plan: Add new unit tests
Reviewed By: desertfire
Differential Revision: D31634044
fbshipit-source-id: a042d54f06fbc480acfc63c18d43cb6fceb6fea5
