[Longformer] Notebook to train Longformer
This PR adds a community notebook that demonstrates how we pretrained Longformer starting from the RoBERTa checkpoint. The same procedure can be followed to convert other existing pretrained models into their Long version.

@patrickvonplaten, @patil-suraj, it would be great if you also check the notebook. Any comments are welcomed. 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4642?src=pr&el=h1) Report
> Merging [#4642](https://codecov.io/gh/huggingface/transformers/pull/4642?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5e737018e1fcb22c8b76052058279552a8d6c806&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4642/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4642?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4642   +/-   ##
=======================================
  Coverage   77.19%   77.19%           
=======================================
  Files         128      128           
  Lines       21021    21021           
=======================================
  Hits        16228    16228           
  Misses       4793     4793           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4642?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4642/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.47% <0.00%> (+0.23%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4642?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4642?src=pr&el=footer). Last update [5e73701...b86c516](https://codecov.io/gh/huggingface/transformers/pull/4642?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>BTW @patil-suraj - you can also add your longformer squad notebook to the community notebooks if you want<|||||>Made a copy and added some comments here: https://colab.research.google.com/drive/1kS2yerrLwLnc-hM6PlisFVaJT98kUKRV#scrollTo=4TTSvW8MlKJJ 

I think the use case is really nice! For better engagement with the notebook, I think it can be polished a bit in terms of small code refactoring / better descriptions. I left some TODO: there as suggestions :-) <|||||>@patrickvonplaten, thanks for the review, the notebook is much better now after incorporating your suggestions. <|||||>Looks great merging!