Avoid failure in `check_repo.py` due to missing backends
# What does this PR do?

My 2 PRs #21903 and #21909 forgot to check missing backends just like `check_all_models_are_auto_configured`, and it could fail on users env where some backends are missinig.

This PR fixes this by checking missing backends. Sorry about that.

**Remark**

```
from transformers.models.auto.modeling_flax_auto import FLAX_MODEL_MAPPING_NAMES
```
this worked even without backend jax/flax. That's why I missed.
_The documentation is not available anymore as the PR was closed or merged._