GEDDataset num_classes request leads to AttributeError

🐛 Bug

When loaded a GEDDataset and querying the num_classes as descriped in the tutorial (dataset.num_classes) an AttributeError: 'NoneType' object has no attribute 'dim'-error is thrown.

To Reproduce

Steps to reproduce the behavior:

1. Load the IMDBMulti dataset with GEDDataset
2. Ask the num_classes.

Code for reproducing error:

from torch_geometric.datasets import GEDDataset
dataset = GEDDataset(root='/datasets/IMDBMulti', name="IMDBMulti")
print(dataset.num_classes)

Expected behavior

I would have expected a return of the number of classes: 3.

Environment

• OS: Ubuntu 18.04 in Docker
• Python version: 3.6.5
• PyTorch version: 1.4.0
• CUDA/cuDNN version: 10.1.243 / 7.6.5.32
• GCC version: 7.5.0
• Any other relevant information: Basis of our Dockerfile is from your provided dockerfile.

Additional context

I noticed that in the examples, the ground truth is saved in the key 'y'.

data = dataset[0]
data.keys()

would return something like ['edge_index', 'y']

The GEDDataset returns ['edge_index', 'i'].

Also tested with the ALKANE dataset, same problem.

[rusty1s]

The i refers to the index of the graph in the dataset, so that you can access the GED between two pairs of graphs via dataset.ged[data1.i, data2.i]. I think we should mention this in the documentation.

[rusty1s]

You can access them via data.x.