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

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

jobs/process/BaseSDTrainProcess.py

@@ -2371,7 +2371,11 @@ class BaseSDTrainProcess(BaseTrainProcess):
                 # todo improve this logic to send one of each through if we can buckets and batch size might be an issue
                 is_reg_step = False
                 is_save_step = self.save_config.save_every and self.step_num % self.save_config.save_every == 0
-                is_sample_step = self.sample_config.sample_every and self.step_num % self.sample_config.sample_every == 0
+                is_sample_step = (
+                    self.sample_config.sample_every
+                    and self.step_num >= self.sample_config.sample_start_step
+                    and self.step_num % self.sample_config.sample_every == 0
+                )
                 if self.train_config.disable_sampling:
                     is_sample_step = False