Move import out of ProcMesh.__del__ (#472)

Summary:
Pull Request resolved: #472

You cannot import a module during the Python runtime shutdown, which leads to an
error if any ProcMesh objects are being garbage collected without being stopped.
Move this import to the top level, it's not important to import it lazily.

Reviewed By: zdevito, colin2328

Differential Revision: D77973599

fbshipit-source-id: 0f90785b3b1f53efc0a5b0f5664cc7d42d295071

Author: dulinriley

python/monarch/_src/actor/proc_mesh.py

@@ -8,6 +8,7 @@
 
 import os
 import sys
+import warnings
 from contextlib import AbstractContextManager
 
 from typing import (
@@ -275,8 +276,6 @@ async def __aexit__(
     # Finalizer to check if the proc mesh was closed properly.
     def __del__(self) -> None:
         if not self._stopped:
-            import warnings
-
             warnings.warn(
                 f"unstopped ProcMesh {self!r}",
                 ResourceWarning,