community[patch]: Add "model" attribute to the payload sent to Ollama in `ChatOllama` (#20354)

Example Ollama API calls:

Request without "model":
```
curl --location 'http://localhost:11434/api/chat' \
--header 'Content-Type: application/json' \
--data '{
  "messages": [
    {
      "role": "user",
      "content": "What is the capitol of PA?"
    }
  ],
  "stream": false
}'
```
Response:
```
{"error":"model is required"}
```

Request with "model":
```
curl --location 'http://localhost:11434/api/chat' \
--header 'Content-Type: application/json' \
--data '{
  "model": "openchat",
  "messages": [
    {
      "role": "user",
      "content": "What is the capitol of PA?"
    }
  ],
  "stream": false
}'
```

Response:
```
{
  "eval_duration" : 733248000,
  "created_at" : "2024-04-11T23:04:08.735766843Z",
  "model" : "openchat",
  "message" : {
    "content" : " The capital city of Pennsylvania is Harrisburg.",
    "role" : "assistant"
  },
  "total_duration" : 3138731168,
  "prompt_eval_count" : 25,
  "load_duration" : 466562959,
  "done" : true,
  "prompt_eval_duration" : 1938495000,
  "eval_count" : 10
}
```

libs/community/langchain_community/chat_models/ollama.py

@@ -156,6 +156,7 @@ class ChatOllama(BaseChatModel, _OllamaCommon):
         **kwargs: Any,
     ) -> Iterator[str]:
         payload = {
+            "model": self.model,
             "messages": self._convert_messages_to_ollama_messages(messages),
         }
         yield from self._create_stream(
@@ -169,6 +170,7 @@ class ChatOllama(BaseChatModel, _OllamaCommon):
         **kwargs: Any,
     ) -> AsyncIterator[str]:
         payload = {
+            "model": self.model,
             "messages": self._convert_messages_to_ollama_messages(messages),
         }
         async for stream_resp in self._acreate_stream(