Fix for empty pd.DataFrame in WebQSPDataset

[puririshi98]

this last minute commit broke things: e331dcd

the changes to , columns=... is for

Traceback (most recent call last):
  File "/opt/pyg/pytorch_geometric/examples/llm/g_retriever.py", line 262, in <module>
    train(
  File "/opt/pyg/pytorch_geometric/examples/llm/g_retriever.py", line 132, in train
    train_dataset = WebQSPDataset(path, split='train')
  File "/usr/local/lib/python3.10/dist-packages/torch_geometric/datasets/web_qsp_dataset.py", line 145, in __init__
    super().__init__(root, force_reload=force_reload)
  File "/usr/local/lib/python3.10/dist-packages/torch_geometric/data/in_memory_dataset.py", line 81, in __init__
    super().__init__(root, transform, pre_transform, pre_filter, log,
  File "/usr/local/lib/python3.10/dist-packages/torch_geometric/data/dataset.py", line 115, in __init__
    self._process()
  File "/usr/local/lib/python3.10/dist-packages/torch_geometric/data/dataset.py", line 262, in _process
    self.process()
  File "/usr/local/lib/python3.10/dist-packages/torch_geometric/datasets/web_qsp_dataset.py", line 202, in process
    nodes.node_attr = nodes.node_attr.fillna("")
  File "/usr/local/lib/python3.10/dist-packages/pandas/core/generic.py", line 6299, in __getattr__
    return object.__getattribute__(self, name)
AttributeError: 'DataFrame' object has no attribute 'node_attr'

then the LongTensor change is for

    return scatter(edge_attr, col, 0, num_nodes, fill_value)
  File "/usr/local/lib/python3.10/dist-packages/torch_geometric/utils/_scatter.py", line 79, in scatter
    count.scatter_add_(0, index, src.new_ones(src.size(dim)))
RuntimeError: scatter(): Expected dtype int64 for index

this is the handle the edgecase where the Question Answer pair has no knowledge graph associated

[akihironitta]

LGTM if the change is manually tested (as we don't have unit tests for torch_geometric.datasets)!