Effectively allow `encoder_outputs` input to be a tuple in pix2struct
Be consistent with the type hint https://github.com/huggingface/transformers/blob/fabe17a726bbf6081cfbcc975d8ac451a81f3e2d/src/transformers/models/pix2struct/modeling_pix2struct.py#L1656 (which should rather be `Optional[Union[Tuple[Tuple[torch.FloatTensor]], OrderedDict]]` IMO) and follow what is done with other architectures.

Otherwise, currently, with `return_dict=True`, an error is raised as we later try to access `encoder_outputs.last_hidden_state`: https://github.com/huggingface/transformers/blob/fabe17a726bbf6081cfbcc975d8ac451a81f3e2d/src/transformers/models/pix2struct/modeling_pix2struct.py#L1768-L1781

This is blocking for the ONNX export support.
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