Fix bug about can not join current thread during graph unloading (#177)

* Fix bug about can not join current thread during graph unloading
* format code

python/graphscope/framework/graph.py

@@ -288,7 +288,13 @@ def unload(self):
                 self._interactive_instance_launching_thread is not None
                 and self._interactive_instance_launching_thread.is_alive()
             ):
-                self._interactive_instance_launching_thread.join()
+                # join raises a RuntimeError if an attempt is made to join the current thread.
+                # this exception occurs when a object collected by gc mechanism contains a running thread.
+                if (
+                    threading.current_thread()
+                    != self._interactive_instance_launching_thread
+                ):
+                    self._interactive_instance_launching_thread.join()
             self._close_interactive_instances()
         except Exception as e:
             logger.error("Failed to close interactive instances: %s" % e)