Fix missing f-string prefix in error message (#6684)

Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>

torchvision/models/detection/roi_heads.py

@@ -746,7 +746,7 @@ class RoIHeads(nn.Module):
                 if not t["boxes"].dtype in floating_point_types:
                     raise TypeError(f"target boxes must of float type, instead got {t['boxes'].dtype}")
                 if not t["labels"].dtype == torch.int64:
-                    raise TypeError("target labels must of int64 type, instead got {t['labels'].dtype}")
+                    raise TypeError(f"target labels must of int64 type, instead got {t['labels'].dtype}")
                 if self.has_keypoint():
                     if not t["keypoints"].dtype == torch.float32:
                         raise TypeError(f"target keypoints must of float type, instead got {t['keypoints'].dtype}")