Move torch.cuda's atfork handler into C++ (#29101)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/23401
We cannot rely on `multiprocessing.util.register_after_fork` since it is only
called for processes created by the `multiprocessing` module and not `os.fork()`.
Moving to `pthread_atfork` does always get called. However, I don't think it's safe to call python functions inside of the `atfork` handler so the python code has to be a bit more careful when checking `_initialized`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29101
Differential Revision: D18355451
Pulled By: ezyang
fbshipit-source-id: 4d4253a3669796212c099dad4e5bdfdb0df40469
