LongformerForTokenClassification
This PR adds `LongformerForTokenClassification`

@patrickvonplaten  @ibeltagy 
do we need any automatic global attention here ?
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4638?src=pr&el=h1) Report
> Merging [#4638](https://codecov.io/gh/huggingface/transformers/pull/4638?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/96f57c9ccb6363623005fb3f05166dfd7acb3f53&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `96.55%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4638/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4638?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4638      +/-   ##
==========================================
+ Coverage   77.39%   77.43%   +0.04%     
==========================================
  Files         128      128              
  Lines       20989    21018      +29     
==========================================
+ Hits        16244    16275      +31     
+ Misses       4745     4743       -2     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4638?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/4638/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.13% <ø> (ø)` | |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/4638/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `73.80% <ø> (ø)` | |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/4638/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `96.83% <96.55%> (-0.03%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4638/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/4638/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.78% <0.00%> (+1.37%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4638?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/4638?src=pr&el=footer). Last update [96f57c9...c8bcd05](https://codecov.io/gh/huggingface/transformers/pull/4638?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>No. We didn't use any global attention for token classification tasks. It is possible that certain choices of global attention will improve results, but it is task-specific and better be left to the user. <|||||>Great! Then I think it can be merged.<|||||>Awesome! LGTM