Update default ouput extension in optimize_for_mobile.cc (#45598)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45598
.bc is causing issues on Android. Let's switch to .ptl.
Test Plan: CI
Reviewed By: kimishpatel
Differential Revision: D24026180
fbshipit-source-id: 9f252f3652d748bccb19dc61a783d693e171b2c6
