[Community notebooks] add longformer-for-qa notebook
This PR adds a community notebook to showcase how to fine-tune Longformer for QA task.

@ibeltagy  @patrickvonplaten Please provide feedback if you think this notebook can be further improved.

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4652?src=pr&el=h1) Report
> Merging [#4652](https://codecov.io/gh/huggingface/transformers/pull/4652?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5e737018e1fcb22c8b76052058279552a8d6c806&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4652/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4652?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4652      +/-   ##
==========================================
- Coverage   77.19%   77.19%   -0.01%     
==========================================
  Files         128      128              
  Lines       21021    21021              
==========================================
- Hits        16228    16227       -1     
- Misses       4793     4794       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4652?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4652/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.44% <0.00%> (-0.42%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4652/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.50% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4652/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/4652?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/4652?src=pr&el=footer). Last update [5e73701...88eab9f](https://codecov.io/gh/huggingface/transformers/pull/4652?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>One small comment, the model name was changed in a recent commit to `allenai/longformer-base-4096'`.
<|||||>The notebook looks great - thanks @patil-suraj ! Apart from @ibeltagy's suggestion, I think it's great!<|||||>Great, thank you! I've updated the model paths.