Add `finetuned_from` property in the autogenerated model card
# What does this PR do?

We already extract the info but it wasn't properly put in the metadata. This PR adds it.
cc @julien-c @osanseviero for the exact name of the tag to use.
_The documentation is not available anymore as the PR was closed or merged._<|||||>What did you have in mind exactly? The only text added is 
```
"This model is a fine-tuned version of f" [{self.finetuned_from}](https://huggingface.co/{self.finetuned_from}) on "
```
which doesn't contain the `finetuned_from` tag.<|||||>@sgugger i would rename the variable everywhere<|||||>That's a breaking change in Transformers for cosmetic reasons only, so a big no no from my side.<|||||>I would go ahead with this PR as-is :fire: 