fix max_length calculating using cu_seq_lens
Author: KKZ20
Pull request: yes

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Comments:
cc @zucchini-nlp 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_39341). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Let's merge it to unblock PR for qwen2-vl with packing