add mbart to automodel for masked lm
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes #9653

Bart and MBart are the only Encoder-Decoder models that can do mask-filling -> so add MBart also to `AutoModelForMaskedLM`

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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 albert, bert, XLM: @LysandreJik
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Benchmarks: @patrickvonplaten
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 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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 Text Generation: @patrickvonplaten, @TevenLeScao
 Blenderbot, Bart, Marian, Pegasus: @patrickvonplaten
 T5: @patrickvonplaten
 Rag: @patrickvonplaten, @lhoestq
 EncoderDecoder: @patrickvonplaten
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