Add DeBERTa to MODEL_FOR_PRETRAINING_MAPPING
Author: jeswan
Pull request: yes

# What does this PR do?

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This PR adds DebertaForMaskedLM to MODEL_FOR_PRETRAINING_MAPPING since DeBERTa is currently missing from this dict.


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- [ ] Did you write any new necessary tests?

@patrickvonplaten
@sgugger 

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Models:

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Comments:
@LysandreJik Added!

Very cool, thanks! Will merge once all the tests are green.