how to finetune with PreTrainedEncoderDecoder
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## Details
<!-- Description of your issue --> I am trying to run seq2seq within the same language (say it is English). I was trying to use PreTrainedEncoderDecoder (BERT, BERT), was trying to use BERT and GPT2 however, looks like it does not support this combination yet. 
I am trying to understand what the forward function does in the class PreTrainedEncoderDecoder, and how can we use it training my dataset.
Also how can we use it at prediction time since forward need to have both decode_input_ids and encode_input_ids. I do not think we will have decode_input_ids at prediction time. Thank you

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