Added BigBirdPegasus onnx config
Author: nandwalritik
Pull request: yes

# What does this PR do?

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Added BigBirdPegasus OnnxConfig to make this model available for conversion.
Sorry in my last pull request I forced push with wrong commit hash, which closed that PR. So I created a new one with same changes. Please consider.
@ChainYo @lewtun 

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```
RUN_SLOW=1 pytest tests/onnx/test_onnx_v2.py -s -k "bigbird_pegasus"
```

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Oh you closed the other one?

> Oh you closed the other one?

While rebasing, I mistakenly forced pushed with wrong commit hash which reset the head  of my branch to latest commit from main branch. All my previous commits were lost and pull request got closed automatically, I didn't had any other option other than creating a new pull request.