Update Tatoeba conversion
# 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.

This was previously reviewed by @patrickvonplaten [here](https://github.com/huggingface/transformers/pull/12192), but we didn't get around to merging it because of an unclean commit history, so I made a fresh branch now to be extra sure.

## 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?

@patrickvonplaten 
Great, thanks, and thanks @patil-suraj, I'll try to come back to those at some point.