Preserve the order of loaded openers in selfchat (#3923)

parlai/tasks/self_chat/worlds.py

@@ -40,18 +40,21 @@ def load_openers(opt) -> Optional[List[str]]:
     task_world = create_task(task_opt, task_agent)
 
     # run through task data, collecting all first messages
-    openers = set()
+    openers = []
     is_first_turn = True
     while not task_world.epoch_done():
         task_world.parley()
         msg = task_world.get_acts()[0]
         # add only the first message in the episode
         if is_first_turn and msg.get('text'):
-            openers.add(msg['text'])
+            openers.append(msg['text'])
         is_first_turn = msg.get('episode_done', False)
 
+    # remove duplicates while preserving the ordering of the loaded openers
+    openers = list(dict.fromkeys(openers))
+
     print(f'[ loaded {len(openers)} openers ]')
-    return list(openers)
+    return openers
 
 
 def load_openers_from_file(filepath: str) -> List[str]: