feat: add option to switch the sigma schedule (#51)

Concretely, this allows switching to the "Karras" schedule from the
Karras et al 2022 paper, equivalent to the samplers marked as "Karras"
in the AUTOMATIC1111 WebUI. This choice is in principle orthogonal to
the sampler choice and can be given independently.

stable-diffusion.h

@@ -25,6 +25,13 @@ enum SampleMethod {
     N_SAMPLE_METHODS
 };
 
+enum Schedule {
+    DEFAULT,
+    DISCRETE,
+    KARRAS,
+    N_SCHEDULES
+};
+
 class StableDiffusionGGML;
 
 class StableDiffusion {
@@ -36,7 +43,7 @@ class StableDiffusion {
                     bool vae_decode_only = false,
                     bool free_params_immediately = false,
                     RNGType rng_type = STD_DEFAULT_RNG);
-    bool load_from_file(const std::string& file_path);
+    bool load_from_file(const std::string& file_path, Schedule d = DEFAULT);
     std::vector<uint8_t> txt2img(
         const std::string& prompt,
         const std::string& negative_prompt,