Add DiffusionPipeline support and auto mode retry logic

- Add DiffusionPipeline to PIPELINE_CLASS_MAP for generic model loading
- Add fallback to DiffusionPipeline for unknown pipeline classes
- Add return_all parameter to select_best_model() for getting all candidates
- Store alternative models in auto mode for retry support
- Implement retry logic when model loading fails in auto mode
- Retry up to 3 times with alternative models before failing
- Add debug output for model loading troubleshooting
- Improve error messages with troubleshooting hints