use spawn as multiprocessing start method (#564)

Summary:
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
ghstack-source-id: 296986085

Reviewed By: highker

Differential Revision: D78465499

fbshipit-source-id: e62353ffc6d5b45de688e9b846c702fce64a29f9

Author: suo

python/monarch/_src/actor/bootstrap_main.py

@@ -11,6 +11,7 @@
 import asyncio
 import importlib.resources
 import logging
+import multiprocessing
 import os
 import sys
 
@@ -64,4 +65,7 @@ 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