Update cohere_chat.py

Add chat history to cohere chat call that was previously being dropped.

litellm/llms/cohere_chat.py

@@ -235,6 +235,7 @@ def completion(
         optional_params["message"] = most_recent_message
     data = {
         "model": model,
+        "chat_history": = chat_histrory,
         **optional_params,
     }