`Engine::~Engine()` should wait for non-reentrant threads to shutdown (#34529)
Summary:
Because `this` must be valid while `Engine::main_thread` is running, at least for non-reentrant worker threads
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34529
Test Plan: Run `test_api --gtest-filter=ModulesTest.InstanceNorm1d` in a loop
Differential Revision: D20552717
Pulled By: malfet
fbshipit-source-id: a0197671db1b7b1499dda675e43e0826f368bf0d
