[Marian] documentation and AutoModel support
- Adds integration tests for en-fr, fr-en.
- Easier bulk conversion
- remove unused pretrained_model_archive_map constant.
- boilerplate to make AutoModelWithLMHead, AutoTokenizer, AutoConfig work.

### Metrics:

For [fr-en test set](https://object.pouta.csc.fi/OPUS-MT-models/fr-en/opus-2020-02-26.test.txt):
- BLEU score from posted translations: 57.4979
- BLEU score from `MarianMTModel.from_pretrained('Helsinki-NLP/opus-mt-fr-en')`: 57.4817
- no perf change for fp16
- can fit batch_size=512 on 16GB card in fp16
- speed: 89s for 5k examples = 56 examples/second

### Screenshots of Documentation

![image](https://user-images.githubusercontent.com/6045025/81184705-b8f5f700-8f7e-11ea-896f-9571d542cb32.png)
![image](https://user-images.githubusercontent.com/6045025/81184741-c317f580-8f7e-11ea-96f2-a38c89224f7b.png)

[  x] Shows up in TOC:
![image](https://user-images.githubusercontent.com/6045025/81184776-cd39f400-8f7e-11ea-8420-cf5f34eac1d7.png)
<|||||>The documentation looks great!<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4152?src=pr&el=h1) Report
> Merging [#4152](https://codecov.io/gh/huggingface/transformers/pull/4152?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/a01a3fecb4fe203c086d3d7450b76bbcfa035725&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4152/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4152?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4152   +/-   ##
=======================================
  Coverage   77.52%   77.52%           
=======================================
  Files         120      120           
  Lines       19932    19932           
=======================================
  Hits        15453    15453           
  Misses       4479     4479           
```



------

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