Including background class for training

### 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).

### My Question

Hi,

as in current config file we are ignoring background label.

https://github.com/isl-org/Open3D-ML/blob/8ddb67206e4fef55b39eea691ff00d49cef18be5/ml3d/configs/randlanet_toronto3d.yml#L6-L7

1. Is it the reason why we are adding all class labels by 1 during inference in inference script below? If not, what is the reason? I could not understand it.

https://github.com/isl-org/Open3D-ML/blob/8ddb67206e4fef55b39eea691ff00d49cef18be5/examples/vis_pred.py#L41

2. And what does ```Fill "unlabeled" value because predictions have no 0 values.``` mean here below?
https://github.com/isl-org/Open3D-ML/blob/8ddb67206e4fef55b39eea691ff00d49cef18be5/examples/vis_pred.py#L47-L48
It seems, we are just assigning the zero-th index of prediction labels `0`. What does it even mean?

3. I want to train a model with only one class. What do you think is the best way to proceed for the task
- Ignore background class and train on single class? (or)
- Include background class and train it as binary predictor - that will give two probabilities `is it an asset` & `is it not an asset`