[WIP] Lightning glue example
This PR adds an example of using Pytorch Lightning to run the GLUE benchmark. Additionally, I altered the `transformer_base.py` to use auto models and moved it to the example directory so it could be copied in by any script that wishes to use it.

Preferably, the base transformer would have subclasses for the different types of tasks, but I just used a dictionary with a key passed on init instead. (i.e. NER uses `AutoModelForTokenClassification` and GLUE uses `AutoModelForSequenceClassification`).
@srush Can you please take a look?<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3290?src=pr&el=h1) Report
> Merging [#3290](https://codecov.io/gh/huggingface/transformers/pull/3290?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/8320feec09309a94f673e1e7ce2a93da81eb3366&el=desc) will **increase** coverage by `0.18%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3290/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3290?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3290      +/-   ##
==========================================
+ Coverage   77.81%   77.99%   +0.18%     
==========================================
  Files          98       98              
  Lines       16666    16666              
==========================================
+ Hits        12969    12999      +30     
+ Misses       3697     3667      -30     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3290?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3290/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `93.84% <0.00%> (+0.27%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3290/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.47% <0.00%> (+5.00%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3290?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/3290?src=pr&el=footer). Last update [8320fee...dd1b783](https://codecov.io/gh/huggingface/transformers/pull/3290?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Looks excellent. I will let @LysandreJik merge tomorrow, and confirm multi-gpu / TPU work. 

Want to try SQuAD next?<|||||>> Want to try SQuAD next?

Sure, I'll give it a go.