New method converts results from torch tensors to pandas DataFrames with column names.

This PR may partially resolve issue #2703

```python
print(results.pandas().xyxy[0])

         xmin        ymin        xmax        ymax  confidence  class    name
0   57.068970  391.770599  241.383545  905.797852    0.868964    0.0  person
1  667.661255  399.303589  810.000000  881.396667    0.851888    0.0  person
2  222.878387  414.774231  343.804474  857.825073    0.838376    0.0  person
3    4.205386  234.447678  803.739136  750.023376    0.658006    5.0     bus
4    0.000000  550.596008   76.681190  878.669922    0.450596    0.0  person
```

torch example input now shown resized to size=640 and also now a multiple of P6 stride 64 (see #2722 (comment))