Marian tatoeba conversion update
# What does this PR do?

The Helsinki-NLP / Tatoeba NMT models have gone through various
architectural changes, and the old conversion code fails on them. This
commit is something of a rewrite to remedy this, in particular parsing
supplied yaml files rather than README.md files. It needs to be looked
at by someone on the Huggingface side.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@sgugger @sshleifer
cc @patrickvonplaten who did the conversion<|||||>Hey @Traubert,

Thanks a lot for adapting the conversion script! Could you maybe post a link to a Marian Tatoeba model so that I can try it out? 
<|||||>I'll fix the issues you mentioned, and I think the output needs to be made a bit neater by omitting some things. Currently a lot of technical details are copied from the model's yaml description. Is there a huggingface guideline for what model cards should look like?

@patrickvonplaten The converter downloads the models, so you should be able to test like:

```python
from convert_marian_tatoeba_to_pytorch import *
conv = TatoebaConverter()
conv.convert_models(('fin-eng',), dry_run = False)
```

This would result in the converter looking in the metadata from the Tatoeba-Challenge repository, which you are supposed to have available locally, and choosing the best model for that pair. It will then download and convert it, I think in that case this file: https://object.pouta.csc.fi/Tatoeba-MT-models/fin-eng/opus+bt-2021-04-30.zip<|||||>Hm. There's still a failing CI test from isort, but I ran that, committed the change, and on my machine `isort --check-only src/transformers/models/marian/convert_marian_tatoeba_to_pytorch.py` now reports nothing. Any ideas? @sgugger @patrickvonplaten <|||||>Make sure you have the good versions installed, they are all pinned so `pip install -e .[quality]` in the repo should do the trick.<|||||>> Make sure you have the good versions installed, they are all pinned so `pip install -e .[quality]` in the repo should do the trick.

Thanks - also I didn't realise that `make style` was doing something more than plain `isort`, so now I committed another styled-by-make-style version.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Sorry for being so unresponsive on this PR @Traubert - do you think it could be possible to open a copy of the PR with a clean git commit history? Think some external git commits got accidentally merged into this PR.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.