Add padding capabilities to HeteroData.to_homogeneous() (#7374)

useful for putting randomly generated FakeHeteroDataset data into a
fused GNN like RGCNConv, otherwise the fakeheterodataset usually has x's
w/ diff num of features and the resulting Data would not have any node
features w/o this PR

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: rusty1s <matthias.fey@tu-dortmund.de>

CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Added
 
+- Added padding capabilities to `HeteroData.to_homogeneous()` in case feature dimensionalities do not match ([#7374](https://github.com/pyg-team/pytorch_geometric/pull/7374))
 - Added an optional `batch_size` argument to `fps`, `knn`, `knn_graph`, `radius` and `radius_graph` ([#7368](https://github.com/pyg-team/pytorch_geometric/pull/7368))
 - Added `PrefetchLoader` capabilities ([#7376](https://github.com/pyg-team/pytorch_geometric/pull/7376), [#7378](https://github.com/pyg-team/pytorch_geometric/pull/7378), [#7383](https://github.com/pyg-team/pytorch_geometric/pull/7383))
 - Added an example for hierarichial sampling ([#7244](https://github.com/pyg-team/pytorch_geometric/pull/7244))

test/data/test_hetero_data.py

@@ -489,6 +489,22 @@ def test_to_homogeneous_and_vice_versa():
     assert out['author'].num_nodes == 200
 
 
+def test_to_homogeneous_padding():
+    data = HeteroData()
+    data['paper'].x = torch.randn(100, 128)
+    data['author'].x = torch.randn(50, 64)
+
+    out = data.to_homogeneous()
+    assert len(out) == 2
+    assert out.node_type.size() == (150, )
+    assert out.node_type[:100].abs().sum() == 0
+    assert out.node_type[100:].sub(1).abs().sum() == 0
+    assert out.x.size() == (150, 128)
+    assert torch.equal(out.x[:100], data['paper'].x)
+    assert torch.equal(out.x[100:, :64], data['author'].x)
+    assert out.x[100:, 64:].abs().sum() == 0
+
+
 def test_hetero_data_to_canonical():
     data = HeteroData()
     assert isinstance(data['user', 'product'], EdgeStorage)

torch_geometric/data/hetero_data.py

@@ -828,10 +828,20 @@ def fill_dummy_(stores: List[BaseStorage],
 
         def _consistent_size(stores: List[BaseStorage]) -> List[str]:
             sizes_dict = get_sizes(stores)
-            return [
-                key for key, sizes in sizes_dict.items()
-                if len(sizes) == len(stores) and len(set(sizes)) == 1
-            ]
+            keys = []
+            for key, sizes in sizes_dict.items():
+                # The attribute needs to exist in all types:
+                if len(sizes) != len(stores):
+                    continue
+                # The attributes needs to have the same number of dimensions:
+                lengths = set([len(size) for size in sizes])
+                if len(lengths) != 1:
+                    continue
+                # The attributes needs to have the same size in all dimensions:
+                if len(sizes[0]) != 1 and len(set(sizes)) != 1:
+                    continue
+                keys.append(key)
+            return keys
 
         if dummy_values:
             self = copy.copy(self)
@@ -855,6 +865,17 @@ def _consistent_size(stores: List[BaseStorage]) -> List[str]:
                 continue
             values = [store[key] for store in self.node_stores]
             dim = self.__cat_dim__(key, values[0], self.node_stores[0])
+            dim = values[0].dim() + dim if dim < 0 else dim
+            # For two-dimensional features, we allow arbitrary shapes and pad
+            # them with zeros if necessary in case their size doesn't match:
+            if values[0].dim() == 2 and dim == 0:
+                _max = max([value.size(-1) for value in values])
+                for i, v in enumerate(values):
+                    if v.size(-1) < _max:
+                        values[i] = torch.cat(
+                            [v, v.new_zeros(v.size(0), _max - v.size(-1))],
+                            dim=-1,
+                        )
             value = torch.cat(values, dim) if len(values) > 1 else values[0]
             data[key] = value