denoiser.hpp: simplify DPM++ (2S) Ancestral

[vmobilis]

DPMPP2S_A:

stable-diffusion.cpp/denoiser.hpp

Line 686 in 10c6501

if (sigma_down == 0) {

Current formula:

if (sigma_down == 0) {
    d = (x - denoised) / sigmas[i];
    dt = sigma_down - sigmas[i];
    x += d * dt;
}

Since sigma_down == 0
=> dt == -sigmas[i]
=> d * dt == (x - denoised) / sigmas[i] * (-sigmas[i]) == denoised - x
=> x = x + denoised - x
=> x = denoised

Proof:

Before	After

Images are almost same.
And thank you for the program.