Distributed eval: SequentialDistributedSampler + gather all results
As discussed in #3944

In our [**`Trainer`**](https://github.com/huggingface/transformers/blob/master/src/transformers/trainer.py), in torch.distributed mode, eval/predict are currently running on a single node. This PR attempts to fix this.

This relies on:
- a custom `DistributedSampler` named SequentialDistributedSampler that makes it easier to collate inference results at the end of the loop
- some tricky code to gather numpy arrays back to a single node (or to all of them). 

I might be missing something simpler here. For instance in `pytorch/xla` there seems to be a [built-in function to do this](https://github.com/huggingface/transformers/blob/7b75aa9fa55bee577e2c7403301ed31103125a35/src/transformers/trainer.py#L656-L659). 

Please let me know if that's the case 😁
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4243?src=pr&el=h1) Report
> Merging [#4243](https://codecov.io/gh/huggingface/transformers/pull/4243?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/2d05480174c736399c212b3d60b8774fbbf2b6c2&el=desc) will **decrease** coverage by `0.13%`.
> The diff coverage is `32.05%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4243/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4243?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4243      +/-   ##
==========================================
- Coverage   78.15%   78.02%   -0.14%     
==========================================
  Files         120      120              
  Lines       20053    20105      +52     
==========================================
+ Hits        15673    15686      +13     
- Misses       4380     4419      +39     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4243?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4243/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `39.67% <32.05%> (-1.78%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4243/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `90.48% <0.00%> (-0.13%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4243?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4243?src=pr&el=footer). Last update [2d05480...3e32aea](https://codecov.io/gh/huggingface/transformers/pull/4243?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
