[inductor] force 'fork' method for processes, cleanup (#87411)
To cooperate with other multithreading methods, this
forces the process pool to use 'fork' even if others have set it
diferently. We require fork because otherwise `if __name__ == __main__`
needs to be set which we do not control as a library.
Furthermore this adds code to cleanup worker processes if
the parent exits abnormally (e.g. segfault). Previously we would leave
live but inactive workers around.
cc @jansel @lezcano @fdrocha
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87411
Approved by: https://github.com/soumith, https://github.com/anijain2305
