[Examples] Uniform notations in train_flux_lora

[tomguluson92]

What does this PR do?

Uniform naming problem existed in Train_flux_lora @sayakpaul

Since the in parameter size represents args.resolution, I thus replace the args.resolution inside DreamBoothData with size. And revise some notations such as center_crop.

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[sayakpaul]

Thanks!

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[linoytsaban]

thanks!

[yiyixuxu]

can we run make style?

[HuggingFaceDocBuilderDev]

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