separate model card git push from the rest
# What does this PR do?

- After model card metadata contents validation was deployed to the Hub, we need to ensure transformer's trainer git push are not blocked because of an invalid README.mld yaml.
- as discussed with @julien-c @Pierrci @sgugger and @LysandreJik the first step to match Hub's model card validation system is to avoid failing a whole git push after training, for the only reason that README.md metadata is not valid.
- therefore, I tried in this PR to git push the training result independently from the modelcard update, so that the modelcard update failing does not fail the rest, keeping only logging for README.Md push failures.

- Relates to https://github.com/huggingface/huggingface_hub/pull/326

## Before submitting
- [x] 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?
- [x] 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.
- [x] 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?


For tracking purposes, do we have another issue for `datasets: - null` @elishowk? (which will still fail with this, AFAICT)<|||||>> For tracking purposes, do we have another issue for `datasets: - null` @elishowk? (which will still fail with this, AFAICT)

Yep : https://github.com/huggingface/transformers/issues/13528<|||||>🥳 first PR on transformers, thanks for your help you all !