[wip/s2s] DistributedSortishSampler
This allows sortish sampler logic to work on multiple GPU.

The strategy is 
1) find the indices that the current rank's data loader should be using (like `DistributedSampler`)
2) reorder those using the `SortishSampler` logic.

### Results 
The results on a small MT task are similar to the 1 GPU setting. 
+ 2 GPU, random sampler: 13 mins/epoch  BLEU 8.6
+ 2 GPU, `DistributedSortishSampler`: 10 mins/epoch, BLEU 8.6

In the chart below, you can see that the sortish sampler gets to a higher BLEU score in the same number of minutes (x axis) (because it has finished a full epoch rather than 70% of one).

![image](https://user-images.githubusercontent.com/6045025/92787937-87030000-f377-11ea-822a-69e50debe95d.png)

@sgugger let me know if you want this in Trainer!
Not necessarily the base Trainer, but definitely the `Seq2SeqTrainer` in preparation if you use this sampler often.<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7056?src=pr&el=h1) Report
> Merging [#7056](https://codecov.io/gh/huggingface/transformers/pull/7056?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/514486739cc732ad05549d81bd48c0aa9e03a0f3?el=desc) will **increase** coverage by `0.25%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7056/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7056?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7056      +/-   ##
==========================================
+ Coverage   79.32%   79.58%   +0.25%     
==========================================
  Files         168      168              
  Lines       32172    32172              
==========================================
+ Hits        25522    25605      +83     
+ Misses       6650     6567      -83     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7056?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9sb25nZm9ybWVyLnB5) | `16.37% <0.00%> (-82.31%)` | :arrow_down: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `19.71% <0.00%> (-72.34%)` | :arrow_down: |
| [src/transformers/tokenization\_bert\_generation.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYmVydF9nZW5lcmF0aW9uLnB5) | `39.28% <0.00%> (-55.36%)` | :arrow_down: |
| [src/transformers/optimization.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9vcHRpbWl6YXRpb24ucHk=) | `34.28% <0.00%> (-48.00%)` | :arrow_down: |
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `75.00% <0.00%> (-25.00%)` | :arrow_down: |
| [src/transformers/modeling\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19yb2JlcnRhLnB5) | `75.91% <0.00%> (-21.17%)` | :arrow_down: |
| [src/transformers/tokenization\_roberta.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fcm9iZXJ0YS5weQ==) | `87.67% <0.00%> (-10.96%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `86.87% <0.00%> (-7.18%)` | :arrow_down: |
| [src/transformers/tokenization\_openai.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fb3BlbmFpLnB5) | `82.57% <0.00%> (-1.52%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_fast.py](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfZmFzdC5weQ==) | `92.85% <0.00%> (-1.43%)` | :arrow_down: |
| ... and [16 more](https://codecov.io/gh/huggingface/transformers/pull/7056/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7056?src=pr&el=footer). Last update [5144867...41dca6e](https://codecov.io/gh/huggingface/transformers/pull/7056?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
