Avoid torch.meshgrid user warning (d2l-ai#1174)

Avoids the following user warning:
```python
~/anaconda3/envs/torch/lib/python3.10/site-packages/torch/functional.py:568: UserWarning: torch.meshgrid: in an upcoming release, it will be required to pass the indexing argument. (Triggered internally at  ../aten/src/ATen/native/TensorShape.cpp:2228.)
  return _VF.meshgrid(tensors, **kwargs)  # type: ignore[attr-defined]
```

Author: liyufan

chapter_computer-vision/anchor.md

@@ -109,7 +109,7 @@ def multibox_prior(data, sizes, ratios):
     # 生成锚框的所有中心点
     center_h = (torch.arange(in_height, device=device) + offset_h) * steps_h
     center_w = (torch.arange(in_width, device=device) + offset_w) * steps_w
-    shift_y, shift_x = torch.meshgrid(center_h, center_w)
+    shift_y, shift_x = torch.meshgrid(center_h, center_w, indexing='ij')
     shift_y, shift_x = shift_y.reshape(-1), shift_x.reshape(-1)
 
     # 生成“boxes_per_pixel”个高和宽，