[fx]Allow rewrite a symbolic traced module (#54011)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54011
After symbolic tracing, `fn` seems to already have "forward" in its globals. In this case, `new_keys` would have length of 0 and we take "forward" from `global_dict` directly as `fn_compiled`.
Test Plan: Added a new test in test_fx_experimental.
Reviewed By: ansley
Differential Revision: D27049012
fbshipit-source-id: 7fbeb50ebb717900ff5fc0a8a0925d6a97f5a6dd
