community[patch]: Fix ChatDatabricsk in case that streaming response doesn't have role field in delta chunk (#21897)

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**Description:**
Fix ChatDatabricsk in case that streaming response doesn't have role
field in delta chunk


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---------

Signed-off-by: Weichen Xu <weichen.xu@databricks.com>

libs/community/langchain_community/chat_models/mlflow.py

@@ -174,9 +174,16 @@ class ChatMlflow(BaseChatModel):
         )
         # TODO: check if `_client.predict_stream` is available.
         chunk_iter = self._client.predict_stream(endpoint=self.endpoint, inputs=data)
+        first_chunk_role = None
         for chunk in chunk_iter:
             choice = chunk["choices"][0]
-            chunk = ChatMlflow._convert_delta_to_message_chunk(choice["delta"])
+
+            chunk_delta = choice["delta"]
+            if first_chunk_role is None:
+                first_chunk_role = chunk_delta.get("role")
+            chunk = ChatMlflow._convert_delta_to_message_chunk(
+                chunk_delta, first_chunk_role
+            )
 
             generation_info = {}
             if finish_reason := choice.get("finish_reason"):
@@ -225,8 +232,10 @@ class ChatMlflow(BaseChatModel):
             return ChatMessage(content=content, role=role)
 
     @staticmethod
-    def _convert_delta_to_message_chunk(_dict: Mapping[str, Any]) -> BaseMessageChunk:
-        role = _dict["role"]
+    def _convert_delta_to_message_chunk(
+        _dict: Mapping[str, Any], default_role: str
+    ) -> BaseMessageChunk:
+        role = _dict.get("role", default_role)
         content = _dict["content"]
         if role == "user":
             return HumanMessageChunk(content=content)