qwen_image: fixes for TREAD

[bghira]

This pull request adds and enhances support for the TREAD routing method in the Qwen Image model and transformer, including improved handling of routing logic and expanded test coverage. The main changes are grouped below.

TREAD Integration in Qwen Image Model

• Added a new tread_init method to the QwenImage model that initializes TREAD routing, validates configuration, and sets up the router with the specified routes.

Improved TREAD Routing Logic in Transformer

• Refactored the TREAD routing logic in the QwenImageTransformer2DModel's forward method to pre-normalize route indices (including handling negative indices), and to clearly delineate when to start and end a route using route_ptr and routing_now state variables. This makes the routing process more robust and easier to follow. [1] [2] [3]

Expanded Test Coverage

• Added new tests for TREAD integration and initialization in the Qwen Image transformer and model, ensuring that the router and routes are correctly set and initialized, and that the new tread_init method behaves as expected. [1] [2]