Ensure finalization of tasks in order manager at exit

Use weakref.finalize to ensure synchronization of
outstanding events at exit.

Author: oleksandr-pavlyk

dpctl/utils/_order_manager.py

@@ -1,5 +1,6 @@
 from collections import defaultdict
 from contextvars import ContextVar
+import weakref
 
 from .._sycl_event import SyclEvent
 from .._sycl_queue import SyclQueue
@@ -88,7 +89,14 @@ def __getitem__(self, q: SyclQueue) -> _SequentialOrderManager:
     def clear(self):
         """Clear content of internal dictionary"""
         _local = self._map.get()
+        for v in _local.values():
+            v.wait()
         _local.clear()
 
 
 SequentialOrderManager = SyclQueueToOrderManagerMap()
+
+def _callback(som):
+    som.clear()
+
+f = weakref.finalize(SequentialOrderManager, _callback, SequentialOrderManager)