Use `default_observer` and `default_weight_observer` in tests (#31424)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31424
att
Test Plan:
test_jit.py
Imported from OSS
Differential Revision: D19162368
fbshipit-source-id: 33b95ba643eeeae942283bbc33f7ceda8d14c431
