When trying to use EdgeDataLoader with a subgraph of my original graph, I encounter this error, which doesnt appear with the original graph:

```
DGLError: Caught DGLError in DataLoader worker process 0.
```

I created the subgraph by doing this:

```
train_size = 0.8
test_size = 1-train_size
train_dict= {}
for etype in g.canonical_etypes:
edge_ids = g.edges(form='eid', etype=etype)
train_edges, test_edges = sklearn.model_selection.train_test_split(
edge_ids,
test_size=test_size,
train_size=train_size,
random_state=10,
)
train_dict[etype] = sorted(train_edges)
subgraph = dgl.edge_subgraph(graph=g, edges=train_dict, preserve_nodes=True)
```

How could I fix this? Thanks.