[FX] Fix recursion depth issue on Graph deepcopy (#46669)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46669
Make `Graph`'s deepcopy behavior iterative rather than recursive. This prevents stack overflow issues with very large `Graph`s
Test Plan: Imported from OSS
Reviewed By: suo
Differential Revision: D24455120
Pulled By: jamesr66a
fbshipit-source-id: 5c37db5acabe313b9a7a464bebe2a82c59e4e2e9
