Fixed an issue with abnormal training loss in Dreambooth Flux2.

[tcaimm]

What does this PR do?

The incorrect training logic of Dreambooth Flux2 img2img has been modified, and the issue of abnormal loss during training has been fixed.

The Dreambooth flux2 img2img program experienced an abnormal loss during training. After investigation, it was found that the problem stemmed from incorrect slice information in model_pred and model_input_ids. The aforementioned logic was then fixed.

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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• [ ] Did you write any new necessary tests?

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