Add sample_start_step setting to configure when sampling starts (#949)

Co-authored-by: Jaret Burkett <jaretburkett@gmail.com>

extensions_built_in/concept_replacer/config/train.example.yaml

@@ -52,6 +52,7 @@ config:
       sample:
         sampler: "ddpm" # must match train.noise_scheduler
         sample_every: 100 # sample every this many steps
+        sample_start_step: 0 # start sampling at this step
         width: 512
         height: 512
         prompts: