Fix empty top-down offsets/scales default

Update the PyTorch top-down path to use a correctly structured fallback `offsets_and_scales` value when no detections are found. This keeps the data shape consistent with post-processing expectations and avoids downstream tuple unpacking/type mismatches.

Author: C-Achard

dlclive/pose_estimation_pytorch/runner.py

@@ -210,7 +210,7 @@ def get_pose(self, frame: np.ndarray) -> np.ndarray:
 
             frame_batch, offsets_and_scales = self._prepare_top_down(tensor, detections)
             if len(frame_batch) == 0:
-                offsets_and_scales = [(0, 0), 1]
+                offsets_and_scales = [((0, 0), (1.0, 1.0))]
             tensor = frame_batch  # still CHW, batched
 
         if self.dynamic is not None: