always restore dlopen flag in dyndep (#22958)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22958
When we use `extension_loader.DlopenGuard()` to dyndep or import modules, it sets a `RTLD_GLOBAL` flag, and restores the original flags after the `yield`. However, if the modules is not there, yield will fail, and the flags won't be restored, creating all kinds of symbol conflict problems.
Reviewed By: bddppq
Differential Revision: D16311949
fbshipit-source-id: 7b9ec6d60423ec5e78cae694b66c2f17493840b0
