Add BigBirdPegasus
# What does this PR do?
This PR will add Google's BigBird-Pegasus. Extending #10183

Following checkpoints will be added:

- [x] [bigbird-pegasus-large-pubmed](https://huggingface.co/google/bigbird-pegasus-large-pubmed)
- [x] [bigbird-pegasus-large-arxiv](https://huggingface.co/google/bigbird-pegasus-large-arxiv)
- [x] [bigbird-pegasus-large-bigpatent](https://huggingface.co/google/bigbird-pegasus-large-bigpatent)

It is verified that uploaded models work correctly, see:

- BigBird Pegasus Arxiv: https://colab.research.google.com/drive/1ntBBkiDgccbKwKmOECB8VWEFeFmZebLN?usp=sharing
- BigBird Pegasus BigPatent: https://colab.research.google.com/drive/1RKI0BG3JUy4Hn8VdIzNLE5QduwtaiXYZ?usp=sharing
- BigBird Pegasus Pubmed: https://colab.research.google.com/drive/1LebnFVp4unqZWRx5gez1hVyqR9cibIoH?usp=sharing

Here a notebook showing how well BigBirdPegasus works on long-document summarization: https://colab.research.google.com/github/vasudevgupta7/bigbird/blob/main/notebooks/bigbird_pegasus_evaluation.ipynb

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests), Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] 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 which may be interested in your PR.

@patrickvonplaten
For running conversion script for `BigBirdPegasus`:

```shell
python3 src/transformers/models/bigbird_pegasus/convert_bigbird_pegasus_tf_to_pytorch.py --tf_ckpt_path src/tf_ckpt/bigbird-pegasus-large-arxiv/model.ckpt-300000 --save_dir src/google/bigbird-pegasus-large-arxiv
```

For running conversion script for bigbird-roberta `EncoderDecoderModel`:

```shell
python3 src/transformers/models/bigbird_pegasus/convert_bigbird_roberta_tf_to_pytorch.py --tf_ckpt_path src/tf_ckpt/bigbird-roberta-arxiv/model.ckpt-300000 --save_dir src/google/bigbird-roberta-arxiv
```<|||||>@LysandreJik, yes we are planning to add this [notebook](https://colab.research.google.com/github/vasudevgupta7/bigbird/blob/main/notebooks/bigbird_pegasus_evaluation.ipynb) with a few modifications.<|||||>@patrickvonplaten, Test failing on CircleCi: `tests/test_modeling_bigbird_pegasus.py::BigBirdPegasusStandaloneDecoderModelTest::test_decoder_model_attn_mask_past` is passing for me locally.

Everything else is fixed!!