[Trainer] suppress warning for length-related columns
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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During the robust speech event multiple participants reported that they were confused by the (harmless) warning: 

```
"The following columns in the training set don't have a corresponding argument in `Wav2Vec2ForCTC.forward` and have been ignored: input_length."
```

The reason why this warning shows up is because we pass a dataset column `"input_length"` to the Trainer  so that `group_by_length` is significantly sped-up when the input lengths are automatically passed to the trainer.

Fixes #https://github.com/huggingface/transformers/issues/15366#issuecomment-1024166475

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

I disagree with this. The column is in the dataset (to speed up the preprocessing) but it then ignored by the Trainer, which warrants something. The logging level is not set at warning, but at info, so the user knows there is nothing wrong with it.

I'm fine with rephrasing the message if you think it helps, but we shouldn't silently discard a column of the training dataset.

In most our examples, we don't clean up the datasets to include only the columns the model want and we never had any suer complain about this info.

Fair point! Made the message a tiny bit more explicit

Failing tests are due to REALM.