Fix backward compatibility of Conversation (#26741)

* Fix backward compatibility of Conversation

I ran into a case where an external library was depending on the `new_user_input` field of Conversation. https://github.com/SeldonIO/MLServer/blob/release/1.4.x/runtimes/huggingface/mlserver_huggingface/codecs/utils.py#L37

 

This field was deprecated as part of the refactor, but if `transformers` wants to maintain backwards compatibility for now (which is mentioned in a few comments) then there's a good argument for supporting it. Some comments referred to it as an "internal" property, but it didn't start with `_` as is Python convention, so I think it's reasonable that other libraries were referencing it directly.

It's not difficult to add it to the other supported backwards-compatible properties. In addition, the implementation of `past_user_inputs` didn't actually match the past behavior (it would contain the most recent message as well) so I updated that as well.

* make style

---------
Co-authored-by: Matt <rocketknight1@gmail.com>

src/transformers/pipelines/conversational.py

@@ -155,17 +155,29 @@ class Conversation:
             yield message["role"] == "user", message["content"]
 
     @property
-    def past_user_inputs(self):
+    def _user_messages(self):
         # This is a legacy property for backwards compatibility. It is recommended to just directly access
         # conversation.messages instead.
         return [message["content"] for message in self.messages if message["role"] == "user"]
 
+    @property
+    def past_user_inputs(self):
+        # This is a legacy property for backwards compatibility. It is recommended to just directly access
+        # conversation.messages instead.
+        return self._user_messages[:-1]
+
     @property
     def generated_responses(self):
         # This is a legacy property for backwards compatibility. It is recommended to just directly access
         # conversation.messages instead.
         return [message["content"] for message in self.messages if message["role"] == "assistant"]
 
+    @property
+    def new_user_input(self):
+        # This is a legacy property for backwards compatibility. It is recommended to just directly access
+        # conversation.messages instead.
+        return self._user_messages[-1]
+
 
 @add_end_docstrings(
     PIPELINE_INIT_ARGS,