Add wmt translation example
PR adds translation example for T5. 
It uses the `sacrebleu` BLEU scorer. 

I adapted the README.md a bit so that users are aware that models in official paper were attained with finetuned T5 @craffel 
> long

Yeah we should definitely try to run it on a GPU - will take a look at that :-) <|||||>Not sure whether we need fp16 and multi-gpu training. I think single GPU training is enough and t5 + wmt does not take much memory. But happy to take a look into it if you guys think it's worth it :-) @thomwolf @LysandreJik @julien-c <|||||>Code quality test fails because of unpinned isort library (see https://github.com/huggingface/transformers/pull/3449)<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3428?src=pr&el=h1) Report
> Merging [#3428](https://codecov.io/gh/huggingface/transformers/pull/3428?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/b4fb94fe6d831b17c0df364b2848c80ef3add154?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3428/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3428?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3428   +/-   ##
=======================================
  Coverage   52.51%   52.51%           
=======================================
  Files         100      100           
  Lines       17051    17051           
=======================================
  Hits         8954     8954           
  Misses       8097     8097
```



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