Empty val batch CUDA device fix (#7539)

Verified fix for https://github.com/ultralytics/yolov5/pull/7525#issuecomment-1106081123

val.py

@@ -227,7 +227,7 @@ def run(
 
             if npr == 0:
                 if nl:
-                    stats.append((correct, *torch.zeros((3, 0))))
+                    stats.append((correct, *torch.zeros((3, 0), device=device)))
                 continue
 
             # Predictions