[Feature Request] Add option to disable generating class images.

[vedantroy]

WHAT I have an existing directory of class images, for one of my datasets, the DreamBooth extension correctly does not generate any new class images. For another one of my datasets (which is of the same size), the DreamBooth extension generates class images, and I don't know why.

It would be useful to check a box called "force no class generation" and then DreamBooth would raise an exception instead of generating new class images.

[github-actions[bot]]

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