First good issue

There have been quite some issues/questions with how to use the Encoder-Decoder model, e.g.: #4483 and #15479 . The main reason for this is that the model docs are quite outdated and we could need a nice How-to-guide.

So I think we have two action items here:

1. Improve https://huggingface.co/docs/transformers/v4.17.0/en/model_doc/encoder-decoder#encoder-decoder-models a.k.a.: https://github.com/huggingface/transformers/blob/master/docs/source/model_doc/encoder-decoder.mdx

We should mention here:
a) How to create a model ? We should show how to use the from_encoder_decoder_pretrained(...) and then how to save the model?
b) How to fine-tune this model? We should mention that this model can then be fine-tuned just like any other encoder-decoder model (Bart, T5, ...)
c) Put a big warning that the config values have to be correctly set and how to set them, e.g. read: #15479

This should be an EncoderDecoderModel specific text and be very concise and short.

In a second step, we should then write a How-to-guide that includes much more details.

More than happy to help someone tackle this first good issue