Add chat_template.argilla_chat support for DPO datasets (#3202)

* Add chat_template.argilla_chat support for DPO datasets

  Creates a new chat_template.argilla_chat prompt strategy for handling
  DPO datasets where chosen/rejected fields contain full conversations
  (messages + final response), following the pattern of chatml.argilla_chat
  and llama3.argilla_chat.

  - Add argilla_chat() function to chat_template.py
  - Add chat_template.argilla_chat to RLHF documentation
  - Add test coverage for argilla_chat with multiple tokenizers

  Dataset format:
  {
    "chosen": [
      {"role": "user", "content": "..."},
      {"role": "assistant", "content": "..."}
    ],
    "rejected": [
      {"role": "user", "content": "..."},
      {"role": "assistant", "content": "..."}
    ]
  }

* Fix chat_template.argilla_chat return value contract and add docstring

- Return (transform_fn, dataset_kwargs) tuple instead of bare transform_fn
- Add remove_columns specification for field_chosen and field_rejected
- Add comprehensive docstring with Args/Returns sections
- Update tests to unpack tuple return value

Addresses PR feedback to maintain consistency with chat_template.default()
and properly specify columns to remove after dataset transformation.

* Update tests/prompt_strategies/test_dpo_chat_templates.py

Co-authored-by: Wing Lian <wing.lian@gmail.com>

---------

Co-authored-by: Wing Lian <wing.lian@gmail.com>

docs/rlhf.qmd

@@ -219,6 +219,21 @@ DPO supports the following types with the following dataset format:
 }
 ```
 
+#### chat_template.argilla_chat
+
+```json
+{
+    "chosen": [
+        {"role": "user", "content": "..."},
+        {"role": "assistant", "content": "..."}
+    ],
+    "rejected": [
+        {"role": "user", "content": "..."},
+        {"role": "assistant", "content": "..."}
+    ]
+}
+```
+
 #### chat_template.default
 
 ```yaml