Fix no SIGCHLD checking in DataLoaderIter._shutdown_workers (#19421)
Summary:
Also
1. Bump multiprocessing test timeout following python core tests
2. Fix one type of flakiness in `test_proper_exit`.
3. Add trace reporting when loader process hangs in `test_proper_exit` using `faulthandler`.
3. Give `test_proper_exit` another try.
I'll heavily retest this.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19421
Differential Revision: D15063728
Pulled By: ezyang
fbshipit-source-id: 4e0d992622e11053c44a9ec237b88b9a28a4472c
