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/backend/embeddings/model_patcher.py

@@ -64,7 +64,7 @@ class ModelPatcher:
     def apply_lora_unet(
         cls,
         unet: UNet2DConditionModel,
-        loras: List[Tuple[LoRAModelRaw, float]],
+        loras: Iterator[Tuple[LoRAModelRaw, float]],
     ) -> None:
         with cls.apply_lora(unet, loras, "lora_unet_"):
             yield
@@ -307,7 +307,7 @@ class ONNXModelPatcher:
     def apply_lora_unet(
         cls,
         unet: OnnxRuntimeModel,
-        loras: List[Tuple[LoRAModelRaw, float]],
+        loras: Iterator[Tuple[LoRAModelRaw, float]],
     ) -> None:
         with cls.apply_lora(unet, loras, "lora_unet_"):
             yield