changed the imgsize on enginge/validator to meet the grayscale for prediction

ultralytics/engine/validator.py

@@ -157,7 +157,7 @@ class BaseValidator:
             self.dataloader = self.dataloader or self.get_dataloader(self.data.get(self.args.split), self.args.batch)
 
             model.eval()
-            model.warmup(imgsz=(1 if pt else self.args.batch, 3, imgsz, imgsz))  # warmup
+            model.warmup(imgsz=(1 if pt else self.args.batch, 1, imgsz, imgsz))  # warmup
 
         dt = Profile(), Profile(), Profile(), Profile()
         n_batches = len(self.dataloader)