[FX] Make Tracer.trace() just return a Graph (#45704)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45704
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D24067982
Pulled By: jamesr66a
fbshipit-source-id: c82aa6be504d45e110055a3c4db129d0b9ac3ef5
