to_kitti_format() does not return correct 2d box image label

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D-ML/issues).
- [X] I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### Describe the issue

When running `to_kitti_format()` on the `Object3d` object, the returned output does not match the original 2d bounding box on Kitti

### Steps to reproduce the bug

```python
import pandas as pd
x = pd.read_pickle('KITTI/bboxes.pkl')
x[0].to_kitti_format()
```

Output:
`'Pedestrian -1.00 -1 -0.21 765.82 225.93 875.76 389.62 1.89 0.48 1.20 1.84 1.47 8.41 0.01 1.00'`

### Expected behavior

Expected Output (Should match actual label in the txt file):
`'Pedestrian -1.00 -1 -0.20 712.40 143.00 810.73 307.92 1.89 0.48 1.20 1.84 1.47 8.41 0.01 1.00'`

### Open3D, Python and System information

```markdown
- Operating system: Ubuntu 20.04
- Python version: Python 3.8
- Open3D version: 0.15.1
```