[OACR][NLU] Add aten::str operator (#43573)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43573
We recently updated the Stella NLU model in D23307228, and the App started to crash with `Following ops cannot be found:{aten::str, }`.
Test Plan: Verified by installing the assistant-playground app on Android.
Reviewed By: czlx0701
Differential Revision: D23325409
fbshipit-source-id: d670242868774bb0aef4be5c8212bc3a3f2f667c
