Query about test point labels for the ParisLille3D Dataset

### 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 `main` branch).

### My Question

Hello!

While loading the ParisLille3D dataset, it appears that there are no labels provided for the test point clouds. **Do you know where we can find these semantic segmentation labels for the test point clouds?** Thanks!

## Steps to reproduce
We can observe the distribution of labels across all 10 classes of points in the `val` split of the dataset:
```py
import open3d.ml.torch as ml3d
import pandas as pd

# construct a dataset by specifying dataset_path
dataset = ml3d.datasets.ParisLille3D(dataset_path=DATASET_DIR)

# Get the validation split
val_split = dataset.get_split('val')

# Print the value counts of the label
for idx in range(len(val_split)):
print(dict(pd.Series(val_split.get_data(idx)['label']).value_counts()))
```
Produces the following output:
```
{1: 12042099, 2: 7193259, 9: 917631, 8: 770451, 0: 179841, 5: 115885, 3: 109906, 6: 54818, 7: 11233, 4: 7298}
```

However, we note a "blank" output for all point clouds in the `test` section:
```py
import open3d.ml.torch as ml3d
import pandas as pd

# construct a dataset by specifying dataset_path
dataset = ml3d.datasets.ParisLille3D(dataset_path=DATASET_DIR)

# Get the test split
test_split = dataset.get_split('test')

# Print the value counts of the label
for idx in range(len(test_split)):
print(dict(pd.Series(test_split.get_data(idx)['label']).value_counts()))
```
Produces the following output:
```
{0: 10000000}
{0: 10000000}
{0: 10000000}
```