fix: allow None as graph save_path setting.

src/anemoi/training/train/train.py

@@ -118,14 +118,18 @@ def graph_data(self) -> HeteroData:
 
         Creates the graph in all workers.
         """
-        graph_filename = Path(
-            self.config.hardware.paths.graph,
-            self.config.hardware.files.graph,
-        )
+        if self.config.hardware.files.graph is not None:
+            graph_filename = Path(
+                self.config.hardware.paths.graph,
+                self.config.hardware.files.graph,
+            )
+
+            if graph_filename.exists() and not self.config.graph.overwrite:
+                LOGGER.info("Loading graph data from %s", graph_filename)
+                return torch.load(graph_filename)
 
-        if graph_filename.exists() and not self.config.graph.overwrite:
-            LOGGER.info("Loading graph data from %s", graph_filename)
-            return torch.load(graph_filename)
+        else:
+            graph_filename = None
 
         from anemoi.graphs.create import GraphCreator