Fix dpr<>bart config for RAG
# What does this PR do?

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There was a big silent bug between the interaction of DPR and BERT. Bert added a new config parameter that DPR does not have -> so a "normal" dpr config crashes when used with BERT. This is actually a very big bug and was introduces in #8276 as pointed out by @lhoestq - thanks!

Two things went wrong here. 

1) We should be more careful in general when introducing new config parameters and calling them via `config.<new_parameter>` especially for models like BERT that can be used with other configs.

2) The DPR tests should have could that, but instead of using a normal DPR config, a BERT-like DPR config was used in the tests, which is dangerous because it exactly doesn't catch errors like those.

This PR fixes 1) and 2) by calling the config in the case of the newly introduces parameter only with `getattr(config, <param_name>, <default_value>)` **and** adds the config functionality to DPR as well (DPR also should have this functionality over BERT's positional embedding). IMO `getattr(config, <param_name>, <default_value>)` should be used for models like BERT in general because they could be used and wrapped in many different ways.

Also the DPR test is fixed to use a DPR config instead of a BERT config.


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