refactor(nodes): merge processors

Consolidate graph processing logic into session processor.

With graphs as the unit of work, and the session queue distributing graphs, we no longer need the invocation queue or processor.

Instead, the session processor dequeues the next session and processes it in a simple loop, greatly simplifying the app.

- Remove `graph_execution_manager` service.
- Remove `queue` (invocation queue) service.
- Remove `processor` (invocation processor) service.
- Remove queue-related logic from `Invoker`. It now only starts and stops the services, providing them with access to other services.
- Remove unused `invocation_retrieval_error` and `session_retrieval_error` events, these are no longer needed.
- Clean up stats service now that it is less coupled to the rest of the app.
- Refactor cancellation logic - cancellations now originate from session queue (i.e. HTTP cancel endpoint) and are emitted as events. Processor gets the events and sets the canceled event. Access to this event is provided to the invocation context for e.g. the step callback.
- Remove `sessions` router; it provided access to `graph_executions` but that no longer exists.

invokeai/app/api_app.py

@@ -50,7 +50,6 @@ if True:  # hack to make flake8 happy with imports coming after setting up the c
         images,
         model_manager,
         session_queue,
-        sessions,
         utilities,
         workflows,
     )
@@ -110,8 +109,6 @@ async def shutdown_event() -> None:
 
 
 # Include all routers
-app.include_router(sessions.session_router, prefix="/api")
-
 app.include_router(utilities.utilities_router, prefix="/api")
 app.include_router(model_manager.model_manager_router, prefix="/api")
 app.include_router(download_queue.download_queue_router, prefix="/api")