[BART] add bart-large-xsum weights
Author: sshleifer
Pull request: yes

- conversion script can now take a path, which is required since this model is not on `torch.hub`. Finetuning with fairseq and then converting to huggingface should work. I also cleaned it up a bit
- Config in S3 is already updated with author-recommended generation parameters:
`(num_beams=6, length_penalty=1., min_length=11, max_length=62`

Context:
These weights are from bart finetuned on the XSum abstractive summarization challenge, which encourages shorter (more abstractive) summaries. It achieves state of the art.

Discussion:
- I propose changing the SummarizationPipeline default to this model in a separate PR, since the summarizations are shorter (and high quality)!

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3422?src=pr&el=h1) Report
> Merging [#3422](https://codecov.io/gh/huggingface/transformers/pull/3422?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/17dceae7a1de5577cd0c07a97dcd5821a08af07c&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3422/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3422?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3422      +/-   ##
==========================================
- Coverage   77.80%   77.79%   -0.01%     
==========================================
  Files         100      100              
  Lines       17051    17051              
==========================================
- Hits        13266    13265       -1     
- Misses       3785     3786       +1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3422?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `100.00% <ø> (ø)` | |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `97.58% <ø> (ø)` | |
| [src/transformers/tokenization\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/3422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmFydC5weQ==) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3422/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.81% <0.00%> (-0.14%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3422?src=pr&el=footer). Last update [17dceae...71fcbc9](https://codecov.io/gh/huggingface/transformers/pull/3422?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).