[fix bug] logits's shape different from label's shape in preprocess_logits_for_metrics
Author: wiserxin
Pull request: yes

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After update my tranformers  4.39.3 --> 4.41.2, training LLM with deepspeed DDP method,  something wrong when it goes to my `preprocess_logits_for_metrics` function: **logits's shape  is different from label's shape**.

Then i discover that in latest trainer [code](https://github.com/huggingface/transformers/blob/377e90392804790d76d818264d39c694a71e07e0/src/transformers/trainer.py#L3811), before passing to preprocess_logits_for_metrics,  logits are padded by function `pad_across_processes` but labels are not. 

It seems like a bug unexpectedly introduced .

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Comments:
cc @muellerzr @SunMarc 

Hi @wiserxin, this bug was indeed introduced by this PR https://github.com/huggingface/transformers/pull/30267. The padding of the labels happens after `preprocess_logits_for_metrics`, hence the issue that you are experiencing. @qubvel, is it safe to put it back where it was ? 

Oh, that's true. @SunMarc yes, should be safe. Alternatively, we can just move the "if" block with labels padding before the "if" block with logits.



> Alternatively, we can just move the "if" block with labels padding before the "if" block with logits

Yeah, I think that's would be better for readability + it was the previous behavior ! Could you make the change @wiserxin ? 

> > Alternatively, we can just move the "if" block with labels padding before the "if" block with logits
> 
> Yeah, I think that's would be better for readability + it was the previous behavior ! Could you make the change @wiserxin ?

sure

@SunMarc @wiserxin let's maybe add a comment about the order of this "if" block to avoid the same bug in the future. :)

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_31447). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

> @SunMarc @wiserxin let's maybe add a comment about the order of this "if" block to avoid the same bug in the future. :)

good idea