fix min-snr implementation (#8466)
* fix min-snr implementation
https://github.com/kohya-ss/sd-scripts/blob/main/library/custom_train_functions.py#L66
* Update train_dreambooth.py
fix variable name mse_loss_weights
* fix divisor
* make style
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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
Co-authored-by: YiYi Xu <yixu310@gmail.com>
