Add valid columns check in _remove_unused_columns method (#31466)
* Add valid columns checking in _remove_unused_columns method
https://github.com/huggingface/datasets/issues/6973#issue-2355517362
https://github.com/huggingface/datasets/issues/6535
https://discuss.huggingface.co/t/indexerror-invalid-key-16-is-out-of-bounds-for-size-0/14298/25
* Update modeling_mixtral.py
* Update modeling_mixtral.py
* Update modeling_mixtral.py