fix node_export.py for new scheduling that does not contain input Producer

aidge_core/export/node_export.py

@@ -39,7 +39,11 @@ class ExportNode(ABC):
             if parent_node is not None:
                 self.inputs_dims.append(self.operator.get_input(idx).dims())
             else:
-                self.inputs_dims.append(None)
+                print(self.operator.get_input(idx))
+                if self.operator.get_input(idx) is not None:
+                    self.inputs_dims.append(self.operator.get_input(idx).dims())
+                else:
+                    self.inputs_dims.append(None)
 
         for idx, child_node in enumerate(self.node.get_children()):
             self.outputs.append(child_node)