isinstance in dvis/dvis.py check breaks for None `torch_tensor`.

isinstance check breaks for None `torch_tensor` in `dvis_vec` .

Fix:
```
    if torch_tensor is not None:
        if isinstance(end_pos, torch_tensor):
            end_pos = end_pos.cpu().numpy()
        if isinstance(start_pos, torch_tensor):
            start_pos = start_pos.cpu().numpy()
```