[examples/text-classification] Fix a bug for using own regression dataset
# What does this PR do?

This PR is to fix https://github.com/huggingface/transformers/issues/9393

Fix a bug in `run_glue.py` so that it can be used for our own dataset of regression tasks.

close #9393 

## 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?
- [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.
- [ ] 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
Thank you for checking the issue and giving the comment.


@sgugger 
Thank you for checking and merging this PR!