Feature Request: Flax Encoder-Decoder Model
# 🚀 Feature request
I am trying to combine ViT with BART/mBART in Flax and it would be great to have something similar to the [EncoderDecoderModel](https://github.com/huggingface/transformers/blob/master/src/transformers/models/encoder_decoder/modeling_encoder_decoder.py) in Flax. 

I am willing to look into it but it will probably take some time (I am not familiar with Flax).

Please let me know if anything can be done regarding this.

Thanks,
Gunjan
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