Adds translation pipeline
The API for translation is as follows: 

```
en_fr_translation = pipeline("translation_en_to_fr")
en_fr_translation("How old are you?")
```

for English to French translation. 

PR adds tests and gives an example in the docstring.
PR builds on #3413 and should be merged after this one.

Example:
![Screenshot from 2020-03-26 11-10-52](https://user-images.githubusercontent.com/23423619/77637172-899ea400-6f55-11ea-83a6-0a476fd33430.png)



# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3419?src=pr&el=h1) Report
> Merging [#3419](https://codecov.io/gh/huggingface/transformers/pull/3419?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/9c683ef01e19c4dc1216dcd1ae3c8e7c44d7b2b9&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `94.44%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3419/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3419?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3419      +/-   ##
==========================================
+ Coverage   77.76%   77.80%   +0.03%     
==========================================
  Files         100      100              
  Lines       16995    17025      +30     
==========================================
+ Hits        13216    13246      +30     
  Misses       3779     3779              
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3419?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/3419/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `98.92% <ø> (ø)` | |
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/3419/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `74.78% <94.44%> (+1.51%)` | :arrow_up: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3419/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `91.86% <0.00%> (+0.13%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3419?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3419?src=pr&el=footer). Last update [9c683ef...a5160a6](https://codecov.io/gh/huggingface/transformers/pull/3419?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
