[s2s] distributed eval in one command
Author: sshleifer
Pull request: yes

### Issue

One GPU command:
```bash
python run_eval.py Helsinki-NLP/opus-mt-en-ro wmt_en_ro/test.source enro_test_translations.txt --reference_path wmt_en_ro/test.target --task translation --score_path mar_test_bleu.json --fp16 --bs 64
# {'bleu': 27.6865, 'n_obs': 1999, 'runtime': 85, 'seconds_per_sample': 0.0425}
```

Multi GPU:
```
python -m torch.distributed.launch --nproc_per_node=2  run_distributed_eval.py --model_name $opus --input_path wmt_en_ro --type_path test --fp16 --save_dir tmp_gen --fp16 --bs 64
python aggregate_distributed_results.py tmp_gen tmp_gen --calc_bleu
cat tmp_gen/metrics.json
# "bleu": 27.7772
```

### New command

```
python -m torch.distributed.launch --nproc_per_node=2  run_distributed_eval.py --model_name Helsinki-NLP/opus-mt-en-ro --data_dir wmt_en_ro --type_path test --save_dir tmp_gen4 --fp16 --bs 64 --task translation
```

### Future PRs

The actual generations differ slightly vs the single gpu implementation, likely because of sortish sampler/leading spaces.

Comments:
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7124?src=pr&el=h1) Report
> Merging [#7124](https://codecov.io/gh/huggingface/transformers/pull/7124?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/206b78d4850d3c6fe85a015654293fc4b803ed7b?el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7124/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7124?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7124      +/-   ##
==========================================
+ Coverage   80.84%   80.86%   +0.02%     
==========================================
  Files         168      168              
  Lines       32284    32284              
==========================================
+ Hits        26099    26108       +9     
+ Misses       6185     6176       -9     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7124?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/data\_collator.py](https://codecov.io/gh/huggingface/transformers/pull/7124/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFfY29sbGF0b3IucHk=) | `92.83% <0.00%> (-0.36%)` | :arrow_down: |
| [src/transformers/modeling\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7124/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iYXJ0LnB5) | `93.93% <0.00%> (+0.16%)` | :arrow_up: |
| [...rc/transformers/data/datasets/language\_modeling.py](https://codecov.io/gh/huggingface/transformers/pull/7124/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL2RhdGFzZXRzL2xhbmd1YWdlX21vZGVsaW5nLnB5) | `94.11% <0.00%> (+1.17%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7124/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `85.46% <0.00%> (+1.25%)` | :arrow_up: |
| [src/transformers/configuration\_bart.py](https://codecov.io/gh/huggingface/transformers/pull/7124/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2JhcnQucHk=) | `94.00% <0.00%> (+4.00%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7124?src=pr&el=footer). Last update [206b78d...be6badf](https://codecov.io/gh/huggingface/transformers/pull/7124?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).