DPOTrainer does not work for multimodal Gemma 4

### Reproduction [This notebook](https://colab.research.google.com/drive/19LT3CkbWs87VLwV1keM6lBMZXYo_UHD3?usp=sharing) outputs: ``` [/usr/local/lib/python3.12/dist-packages/transformers/tokenization_utils_base.py](https://localhost:8080/#) in pad(self, encoded_inputs, padding, max_length, pad_to_multiple_of, padding_side, return_attention_mask, return_tensors, verbose) 2632 # The model's main input name, usually `input_ids`, has been passed for padding 2633 if self.model_input_names[0] not in encoded_inputs: -> 2634 raise ValueError( 2635 "You should supply an encoding or a list of encodings to this method " 2636 f"that includes {self.model_input_names[0]}, but you provided {list(encoded_inputs.keys())}" ValueError: You should supply an encoding or a list of encodings to this method that includes input_ids, but you provided ['prompt', 'chosen', 'rejected', 'mm_token_type_ids', 'prompt_ids', 'chosen_ids', 'rejected_ids'] ``` ### System Info - Platform: Linux-6.6.113+-x86_64-with-glibc2.35 - Python version: 3.12.13 - TRL version: 1.2.0 - PyTorch version: 2.10.0+cu128 - accelerator(s): NVIDIA RTX PRO 6000 Blackwell Server Edition - Transformers version: 5.5.0 - Accelerate version: 1.13.0 - Accelerate config: not found - Datasets version: 4.8.4 - HF Hub version: 1.10.1 - bitsandbytes version: not installed - DeepSpeed version: not installed - Liger-Kernel version: not installed - PEFT version: 0.18.1 - vLLM version: not installed ### Checklist - [x] I have checked that my issue isn't already filed (see [open issues](https://github.com/huggingface/trl/issues?q=is%3Aissue)) - [x] I have included my system information - [x] Any code provided is minimal, complete, and reproducible ([more on MREs](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks)) - [x] Any code provided is properly formatted in code blocks, (no screenshot, [more on code blocks](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-and-highlighting-code-blocks)) - [x] Any traceback provided is complete