feat(nodes): improve types in graph.py

Methods `get_node` and `complete` were typed as returning a dynamically created unions `InvocationsUnion` and `InvocationOutputsUnion`, respectively.

Static type analysers cannot work with dynamic objects, so these methods end up as effectively un-annotated, returning `Unknown`.

They now return `BaseInvocation` and `BaseInvocationOutput`, respectively, which are the superclasses of all members of each union. This gives us the best type annotation that is possible.

Note: the return types of these methods are never introspected, so it doesn't really matter what they are at runtime.

invokeai/app/services/shared/graph.py

@@ -540,7 +540,7 @@ def has_node(self, node_path: str) -> bool:
         except NodeNotFoundError:
             return False
 
-    def get_node(self, node_path: str) -> InvocationsUnion:
+    def get_node(self, node_path: str) -> BaseInvocation:
         """Gets a node from the graph using a node path."""
         # Materialized graphs may have nodes at the top level
         graph, node_id = self._get_graph_and_node(node_path)
@@ -891,7 +891,7 @@ def next(self) -> Optional[BaseInvocation]:
         # If next is still none, there's no next node, return None
         return next_node
 
-    def complete(self, node_id: str, output: InvocationOutputsUnion):
+    def complete(self, node_id: str, output: BaseInvocationOutput) -> None:
         """Marks a node as complete"""
 
         if node_id not in self.execution_graph.nodes: