[examples] unit test for run_bart_sum
Author: sshleifer
Pull request: yes

- add lightning to `examples/requirements.txt`
- first lightning unittest!

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3544?src=pr&el=h1) Report
> Merging [#3544](https://codecov.io/gh/huggingface/transformers/pull/3544?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e5c393dcebf42eaec9c1e1d619b5a7788a2d7c65&el=desc) will **increase** coverage by `0.97%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3544/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3544?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3544      +/-   ##
==========================================
+ Coverage   76.84%   77.81%   +0.97%     
==========================================
  Files         100      100              
  Lines       17064    17064              
==========================================
+ Hits        13112    13279     +167     
+ Misses       3952     3785     -167     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3544?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3544/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.32% <0.00%> (+0.17%)` | :arrow_up: |
| [src/transformers/modeling\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/3544/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19vcGVuYWkucHk=) | `81.54% <0.00%> (+1.34%)` | :arrow_up: |
| [src/transformers/modeling\_xlnet.py](https://codecov.io/gh/huggingface/transformers/pull/3544/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ194bG5ldC5weQ==) | `75.77% <0.00%> (+2.29%)` | :arrow_up: |
| [src/transformers/modeling\_ctrl.py](https://codecov.io/gh/huggingface/transformers/pull/3544/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19jdHJsLnB5) | `98.24% <0.00%> (+2.63%)` | :arrow_up: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/3544/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `95.71% <0.00%> (+10.00%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_pytorch\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3544/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9weXRvcmNoX3V0aWxzLnB5) | `89.93% <0.00%> (+81.20%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3544?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/3544?src=pr&el=footer). Last update [e5c393d...af9dd75](https://codecov.io/gh/huggingface/transformers/pull/3544?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


LGTM from a superficial glance

Planning on merging this April 15 at 7pm EST barring objections.