[FX] Make `graph_copy` examine existing values in val_map (#46104)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/46104
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D24224505
Pulled By: jamesr66a
fbshipit-source-id: ffdf8ea8cb92439f3aacf08b0c0db63ce3a15b8f
