Move num_items_in_batch to correct device before accelerator.gather

[ssharpe42]

What does this PR do?

num_items_in_batch ends up being on CPU when in multi-gpu ddp setting. When trying to compute the total number of items across devices, this fails as seen in #39896. This PR moves it to the correct device before gathering across gpus.

Fixes #39896

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Who can review?

@zach-huggingface, @SunMarc and @qgallouedec

[SunMarc]

Thanks !

[HuggingFaceDocBuilderDev]

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