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/model_manager/load/model_cache/model_cache_default.py

@@ -246,7 +246,7 @@ class ModelCache(ModelCacheBase[AnyModel]):
 
     def move_model_to_device(self, cache_entry: CacheRecord[AnyModel], target_device: torch.device) -> None:
         """Move model into the indicated device."""
-        # These attributes are not in the base ModelMixin class but in derived classes.
+        # These attributes are not in the base ModelMixin class but in various derived classes.
         # Some models don't have these attributes, in which case they run in RAM/CPU.
         self.logger.debug(f"Called to move {cache_entry.key} to {target_device}")
         if not (hasattr(cache_entry.model, "device") and hasattr(cache_entry.model, "to")):