[FX] Fix typo in user stack walk (#80830)
Summary: We used to have `torch/fx/symbolic_trace.py` and it was changed to `torch/fx/_symbolic_trace.py`. This part of the code was not updated.
Test Plan: ci
Differential Revision: D37595542
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80830
Approved by: https://github.com/houseroad
