[Rag] Fix loading of pretrained Rag Tokenizer
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Fixes #7710, #7690 

A bug was introduced in https://github.com/huggingface/transformers/pull/7141#discussion_r503800703 that changes the loading of pre-trained special tokens files. This PR more or less reverts the critical changes so that RAG works again. This can be verified by running this test:

```
RUN_SLOW=1 pytest tests/test_modeling_rag.py::RagModelIntegrationTests::test_rag_sequence_generate_batch
```

A new RAG tokenizer test was added.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dimiss the other checks if that's the case).
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      Pull Request section?
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- [x] Did you write any new necessary tests? 

Running all slow tests to check that nothing breaks.

## Who can review?

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 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
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 Speed and Memory Benchmarks: @patrickvonplaten
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 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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Comments:
Got 2 offline yes! from @thomwolf and @LysandreJik => merging.