Add 'temporal' split to PubMed (Planetoid) dataset.

[nelsonaloysio]

Adds new 'temporal' split to PubMed (Planetoid) dataset, at the moment available from Zenodo and GitHub.

This split was introduced in this paper to evaluate node-level clustering with static and temporal models.

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With the changes introduced by this PR, the following code snippet:

from torch_geometric.datasets import Planetoid
dataset = Planetoid(root='.', name='pubmed', split='temporal')
data = dataset[0]

Returns the dataset with edge-level attribute time, plus edge-level train_mask, val_mask, and test_mask:

Data(x=[19717, 500], edge_index=[2, 88648], y=[19717],
     time=[88648], train_mask=[88648], val_mask=[88648], test_mask=[88648])

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For a detailed description of this dataset split, please refer to the pubmed-temporal repository. :)