Force early initialization of OpenMP in forked children (#29006)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/28389
Intel's OpenMP implementation sets the thread affinity on the first call to an OpenMP function after a fork. By adding an atfork handler we can force this to happen before a user tries to set the affinity in their own DataLoader `worker_init_fn`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29006
Differential Revision: D18782456
Pulled By: ezyang
fbshipit-source-id: ce0b515256da0cf18ceb125e0cdec99a3311bbd3
