make model manager v2 ready for PR review

- Replace legacy model manager service with the v2 manager.

- Update invocations to use new load interface.

- Fixed many but not all type checking errors in the invocations. Most
  were unrelated to model manager

- Updated routes. All the new routes live under the route tag
  `model_manager_v2`. To avoid confusion with the old routes,
  they have the URL prefix `/api/v2/models`. The old routes
  have been de-registered.

- Added a pytest for the loader.

- Updated documentation in contributing/MODEL_MANAGER.md

invokeai/app/invocations/model.py

@@ -17,7 +17,7 @@ from .baseinvocation import (
 
 
 class ModelInfo(BaseModel):
-    key: str = Field(description="Info to load submodel")
+    key: str = Field(description="Key of model as returned by ModelRecordServiceBase.get_model()")
     submodel: Optional[SubModelType] = Field(default=None, description="Info to load submodel")