refactor: simplify DPM++ (2S) Ancestral (leejet#667)

Author: vmobilis

denoiser.hpp

@@ -770,7 +770,6 @@ static void sample_k_diffusion(sample_method_t method,
         } break;
         case DPMPP2S_A: {
             struct ggml_tensor* noise = ggml_dup_tensor(work_ctx, x);
-            struct ggml_tensor* d     = ggml_dup_tensor(work_ctx, x);
             struct ggml_tensor* x2    = ggml_dup_tensor(work_ctx, x);
 
             for (int i = 0; i < steps; i++) {
@@ -785,22 +784,15 @@ static void sample_k_diffusion(sample_method_t method,
                 auto sigma_fn    = [](float t) -> float { return exp(-t); };
 
                 if (sigma_down == 0) {
-                    // Euler step
-                    float* vec_d        = (float*)d->data;
+                    // d = (x - denoised) / sigmas[i];
+                    // dt = sigma_down - sigmas[i];
+                    // x += d * dt;
+                    // => x = denoised
                     float* vec_x        = (float*)x->data;
                     float* vec_denoised = (float*)denoised->data;
 
-                    for (int j = 0; j < ggml_nelements(d); j++) {
-                        vec_d[j] = (vec_x[j] - vec_denoised[j]) / sigmas[i];
-                    }
-
-                    // TODO: If sigma_down == 0, isn't this wrong?
-                    // But
-                    // https://github.com/crowsonkb/k-diffusion/blob/master/k_diffusion/sampling.py#L525
-                    // has this exactly the same way.
-                    float dt = sigma_down - sigmas[i];
-                    for (int j = 0; j < ggml_nelements(d); j++) {
-                        vec_x[j] = vec_x[j] + vec_d[j] * dt;
+                    for (int j = 0; j < ggml_nelements(x); j++) {
+                        vec_x[j] = vec_denoised[j];
                     }
                 } else {
                     // DPM-Solver++(2S)
@@ -809,7 +801,6 @@ static void sample_k_diffusion(sample_method_t method,
                     float h      = t_next - t;
                     float s      = t + 0.5f * h;
 
-                    float* vec_d        = (float*)d->data;
                     float* vec_x        = (float*)x->data;
                     float* vec_x2       = (float*)x2->data;
                     float* vec_denoised = (float*)denoised->data;