Add Summarization to Pipelines
Author: sshleifer
Pull request: yes

Choices:

1) This is not TextGenerationPipeline, so it only supports bart-large-cnn. 
1) It doesn't return the input back to the caller because it is annoyingly long.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3128?src=pr&el=h1) Report
> Merging [#3128](https://codecov.io/gh/huggingface/transformers/pull/3128?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/3814e167d99c4b2e135b250d73deaa3f63ebef0c&el=desc) will **increase** coverage by `0.05%`.
> The diff coverage is `95.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3128/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3128?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3128      +/-   ##
==========================================
+ Coverage   78.02%   78.08%   +0.05%     
==========================================
  Files          98       98              
  Lines       16670    16689      +19     
==========================================
+ Hits        13007    13031      +24     
+ Misses       3663     3658       -5     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3128?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/3128/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `98.91% <ø> (ø)` | |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/3128/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `72.53% <95.00%> (+1.57%)` | :arrow_up: |
| [src/transformers/tokenization\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3128/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHMucHk=) | `91.99% <0.00%> (+0.14%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3128/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.37% <0.00%> (+0.17%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3128/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `94.14% <0.00%> (+0.27%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3128?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/3128?src=pr&el=footer). Last update [3814e16...a123599](https://codecov.io/gh/huggingface/transformers/pull/3128?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


I addressed all comments, and am ready for review @julien-c @thomwolf. 

Is this pipeline ready to go? When I tried to run an example it said that the summarization pipeline is not one of the options.

Hey, @Weilin37 .
Could you send a snippet of code so that I can reproduce your error?
Thanks!

@Weilin37 are you running from master?

> @Weilin37 are you running from master?

Hi, yes it is resolved now. I thought I upgraded but it didn't