feat: add lcm sampler support

This referenced an issue discussion of the stable-diffusion-webui at
https://github.com/AUTOMATIC1111/stable-diffusion-webui/issues/13952, which
may not be too perfect.

stable-diffusion.h

@@ -24,6 +24,7 @@ enum SampleMethod {
     DPMPP2S_A,
     DPMPP2M,
     DPMPP2Mv2,
+    LCM,
     N_SAMPLE_METHODS
 };