Fix AttributeError: 'MMBTConfig' object has no attribute 'use_return_dict'
Author: ahmed-mahran
Pull request: yes

# What does this PR do?

Fixes ```AttributeError: 'MMBTConfig' object has no attribute 'use_return_dict'```
Attributes are copied from `transformers.PreTrainedConfig.__dict__` which does not contain object properties and `use_return_dict` is an object property that's not in the `__dict__`.

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Comments:
The MMBT config is not a `PretrainedConfig`, so there is no point trying to use it as such (the MMBT model in general is completely broken and the original authors did not help/release another opensource version of their models we could use as a guide).

Thanks @sgugger 
Would you please provide a link to this implementation? I'd be interested to proceed on this.

There is none that i know of, that is the problem.

Ah! Any chance we can describe what are the issues with current implementation?

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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