fix custom diffusion training with concept list

[AIshutin]

What does this PR do?

It fixes custom diffusion training script in the setup "concept list + synthetic regularization dataset". Currently, it fails / works incorrectly if class_prompt specified in multi concept scenario. See the linked issue for more information.

Fixes #6709

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@sayakpaul
@nupurkmr9

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[sayakpaul]

Thank you for your contributions!