Enable multi-process DataLoader for dreambooth (#950)

examples/dreambooth/train_dreambooth.py

@@ -502,7 +502,7 @@ def main():
         return batch
 
     train_dataloader = torch.utils.data.DataLoader(
-        train_dataset, batch_size=args.train_batch_size, shuffle=True, collate_fn=collate_fn
+        train_dataset, batch_size=args.train_batch_size, shuffle=True, collate_fn=collate_fn, num_workers=1
     )
 
     # Scheduler and math around the number of training steps.