Fix addition assignment operator issue in GINConv (see https://discus…

…s.pytorch.org/t/use-of-the-addition-assignment-operator-on-a-tensor-leads-to-a-runtimeerror/202898)

molexpress/layers/gin_conv.py

@@ -126,7 +126,7 @@ def call(self, inputs: types.MolecularGraph) -> types.MolecularGraph:
             node_state_updated = self.activation(node_state_updated)
 
         if self.skip_connection:
-            node_state_updated += node_state
+            node_state_updated = node_state_updated + node_state
 
         if self.dropout_rate:
             node_state_updated = self.dropout(node_state_updated)