Fix #15898
Fixes #15898

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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- [ ] Did you write any new necessary tests?


## Who can review?

@sgugger


I guess it's obvious looking at the [glue example] (https://github.com/huggingface/transformers/blob/4cd7ed4b3b7360aef3a9fb16dfcc105001188717/examples/pytorch/text-classification/run_glue.py#L448) referenced in the issue, but just to be sure: is it guaranteed that the actual logits (final output of the models) are always placed in the first position?

I looked at the models of `MODEL_FOR_CAUSAL_LM_MAPPING_NAMES` and it seems a convention. I'm a bit confused because there is a `past_index` arg in `Trainer` that indexes the output of the model, which made me question if the position of the logits is guaranteed, but I guess it's never 0 or there's any other reason I don't know of. You know better than me for sure.


The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15928). All of your documentation changes will be reflected on that endpoint.<|||||>Yes, that's enforced in all the model outputs. The `past_index` is not going to temper with that :-)
Thanks a lot for fixing!