Zero terminal snr (ZSNR)

[daniandtheweb]

ZSNR can improve the generated images colors by making it easier to obtain darker and brighter zones in the image: https://arxiv.org/pdf/2305.08891 (the paper already includes a python function as an example).

This issue is related to #334 as v-prediction is the recommended way to train models in order to get the same result.

An option to enable ZSNR could be very useful at getting better colors in the images.