Wrong label for objects in visualizations

Excellent work! Thanks very much for releasing the codes!
I am trying to evaluate the Interaction with the following command:
```shell
python evaluate.py --config=configs/interactron.yaml
```
But the visualizations seems weird


After checking `tlvis_classes` in `utils/constants.py`, I find hat the index of wrongly-marked label is next to the 'real' label:

In conclusion, directly using category to index its text in `tlvis_classes` leads to the wrong labelling. To address this issue, I add a dummy item to `tlvis_classes`, and the visualisations look fine later.