monarch_env: use main_function for -allocator target (#32)

Summary:
Pull Request resolved: #32

This is the second try after D74718068 got backed out.

Turns out `bootstrap_main` from `_rust_bindings` needs a reference to a running event loop, so it can't simply be passed into `asyncio.run()`:
https://github.com/awestlake87/pyo3-asyncio#a-note-about-asynciorun

We wrap it into a trivial coroutine instead.

Reviewed By: amirafzali

Differential Revision: D75284480

fbshipit-source-id: 24352a212d0490d2f1fe4eb6245212c1abfc1bfa

Author: zsol

python/monarch/bootstrap_main.py

@@ -15,17 +15,17 @@
 
 
 async def main():
+    from monarch._rust_bindings.monarch_hyperactor.bootstrap import bootstrap_main
+
     await bootstrap_main()
 
 
-def invoke_main() -> None:
+def invoke_main():
     # if this is invoked with the stdout piped somewhere, then print
     # changes its buffering behavior. So we default to the standard
     # behavior of std out as if it were a terminal.
     sys.stdout.reconfigure(line_buffering=True)
-    global bootstrap_main
     # TODO: figure out what from worker_main.py we should reproduce here.
-    from monarch._rust_bindings.monarch_hyperactor.bootstrap import bootstrap_main
 
     with (
         importlib.resources.path("monarch", "py-spy") as pyspy,
@@ -39,5 +39,4 @@ def invoke_main() -> None:
 
 
 if __name__ == "__main__":
-    # Do not add code here, it won't be run. Add them to the function called below.
     invoke_main()  # pragma: no cover