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
- `self.offset = self.padding_idx+1`, but add a new parameters will make it more clear

**Reference:**
position Embedding in roberta and other models.

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@patrickvonplaten, @patil-suraj

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_16547). All of your documentation changes will be reflected on that endpoint.

The main reason for the test failure is that the test program does not take into account the case of padding at the beginning of a sentence and that's what my pr deal with!

Hey @Oran-Ac,

Could you clarify a bit what you mean by:

```
Fix bugs in position ids in Bart

the padding_token could be added at the begging of the sentence like gpt2 and roberta
self.offset = self.padding_idx+1, but add a new parameters will make it more clear
```

? 

What is the use case that currently does not work? Can you give an example of this? 

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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