GCN2Conv: Allow for optional edge_weight (#4670)

* initial commit

* changelog

CHANGELOG.md

@@ -23,6 +23,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Added support for graph-level outputs in `to_hetero` ([#4582](https://github.com/pyg-team/pytorch_geometric/pull/4582))
 - Added `CHANGELOG.md` ([#4581](https://github.com/pyg-team/pytorch_geometric/pull/4581))
 ### Changed
+- Allow for optional `edge_weight` in `GCN2Conv` ([#4670](https://github.com/pyg-team/pytorch_geometric/pull/4670))
 - Fixed the interplay between `TUDataset` and `pre_transform` that modify node features ([#4669](https://github.com/pyg-team/pytorch_geometric/pull/4669))
 - Make use of the `pyg_sphinx_theme` documentation template ([#4664](https://github.com/pyg-team/pyg-lib/pull/4664), [#4667](https://github.com/pyg-team/pyg-lib/pull/4667))
 - Refactored reading molecular positions from sdf file for qm9 datasets ([4654](https://github.com/pyg-team/pytorch_geometric/pull/4654))

torch_geometric/nn/conv/gcn2_conv.py

@@ -154,8 +154,8 @@ def forward(self, x: Tensor, x_0: Tensor, edge_index: Adj,
 
         return out
 
-    def message(self, x_j: Tensor, edge_weight: Tensor) -> Tensor:
-        return edge_weight.view(-1, 1) * x_j
+    def message(self, x_j: Tensor, edge_weight: OptTensor) -> Tensor:
+        return x_j if edge_weight is None else edge_weight.view(-1, 1) * x_j
 
     def message_and_aggregate(self, adj_t: SparseTensor, x: Tensor) -> Tensor:
         return matmul(adj_t, x, reduce=self.aggr)