[monarch] use spawn as multiprocessing start method

Pull Request resolved: #564

Always use spawn as a start method, fork is a giant footgun. We were observing hangs in megatron checkpointing, I suspect this is why.

Ref: python/cpython#84559

Differential Revision: [D78465499](https://our.internmc.facebook.com/intern/diff/D78465499/)

**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D78465499/)!
ghstack-source-id: 296751863

Author: suo

python/monarch/_src/actor/bootstrap_main.py

@@ -13,6 +13,7 @@
 import logging
 import os
 import sys
+import multiprocessing
 
 # Import torch to avoid import-time races if a spawned actor tries to import torch.
 try:
@@ -63,4 +64,7 @@ def invoke_main():
 
 
 if __name__ == "__main__":
+    # Ensure that processes started via `multiprocessing` are spawned, not forked.
+    # forking is a terrible default, see: https://github.com/python/cpython/issues/84559
+    multiprocessing.set_start_method("spawn", force=True)
     invoke_main()  # pragma: no cover