[Longformer For Question Answering] Conversion script, doc, small fixes
This PR adds 
- Longformer For Question Answering doc
- Adds the link to the (official) uploaded model: https://huggingface.co/allenai/longformer-large-4096-finetuned-triviaqa
- Some minor refactoring 
- Conversion script

@ibeltagy @patil-suraj 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4593?src=pr&el=h1) Report
> Merging [#4593](https://codecov.io/gh/huggingface/transformers/pull/4593?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b86e42e0ac1b59f21f0eccf351d3346bbe3ed4eb&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4593/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4593?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4593   +/-   ##
=======================================
  Coverage   78.09%   78.09%           
=======================================
  Files         123      123           
  Lines       20624    20625    +1     
=======================================
+ Hits        16106    16108    +2     
+ Misses       4518     4517    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4593?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4593/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4593/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `97.41% <100.00%> (+<0.01%)` | :arrow_up: |
| [src/transformers/tokenization\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4593/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbG9uZ2Zvcm1lci5weQ==) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4593/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.53% <0.00%> (+0.11%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4593?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/4593?src=pr&el=footer). Last update [b86e42e...d8d4187](https://codecov.io/gh/huggingface/transformers/pull/4593?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
