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

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

toolkit/config_modules.py

@@ -80,6 +80,7 @@ class SampleConfig:
     def __init__(self, **kwargs):
         self.sampler: str = kwargs.get('sampler', 'ddpm')
         self.sample_every: int = kwargs.get('sample_every', 100)
+        self.sample_start_step: int = kwargs.get('sample_start_step', 0)
         self.width: int = kwargs.get('width', 512)
         self.height: int = kwargs.get('height', 512)
         self.neg = kwargs.get('neg', False)