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/ip_adapter/ip_adapter.py

@@ -8,7 +8,6 @@ from PIL import Image
 from transformers import CLIPImageProcessor, CLIPVisionModelWithProjection
 
 from invokeai.backend.ip_adapter.ip_attention_weights import IPAttentionWeights
-from invokeai.backend.model_management.models.base import calc_model_size_by_data
 
 from .resampler import Resampler
 
@@ -124,6 +123,9 @@ class IPAdapter:
         self.attn_weights.to(device=self.device, dtype=self.dtype)
 
     def calc_size(self):
+        # workaround for circular import
+        from invokeai.backend.model_manager.load.model_util import calc_model_size_by_data
+
         return calc_model_size_by_data(self._image_proj_model) + calc_model_size_by_data(self.attn_weights)
 
     def _init_image_proj_model(self, state_dict):