Fix bugs in position ids in Bart
Author: Oran-Ac
Pull request: yes

# What does this PR do?

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**Fix bugs in position ids in Bart**
- the padding_token could be added at the begging of the sentence like gpt2 and roberta
- `off_set == padding_idx`, but add a new parameters will make it more clear

**Reference:**
position Embedding in roberta and other models.

## 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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      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@patrickvonplaten, @patil-suraj

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Models:

- albert, bert, xlm: @LysandreJik
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- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
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- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

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- datasets: [different repo](https://github.com/huggingface/datasets)
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