Fix bug in input check for LengthGroupSampler
# What does this PR do?

This commit fixes a bug in the LengthGroupSampler where if
model_input_name is not set, the default value is None instead of
"input_ids"

## 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?

I did not write a test for this, but if necessary I can. Neither this sampler nor the distributed version currently have test coverage for the ValueError this bug raises, but it might not be bad to have.

## Who can review?

@patrickvonplaten
