[fake_tensor] Don't run fallback for fbgemm ops (#106210)
Summary:
This diff also adds more warning messages around allowing a namespace into the
fallback. We need to grandfather in an operator to actually merge this diff.
Test Plan: - existing tests
Differential Revision: D47873841
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106210
Approved by: https://github.com/eellison
