[Bugfix] prevent cli crash (#3132)

Prevent legacy CLI crash caused by removal of convert option
    
- Compensatory change to the CLI that prevents it from crashing when it
tries to import a model.
- Bug introduced when the "convert" option removed from the model
manager.

invokeai/backend/__init__.py

@@ -10,7 +10,7 @@ from .generator import (
     Img2Img,
     Inpaint
 )
-from .model_management import ModelManager
+from .model_management import ModelManager, SDModelComponent
 from .safety_checker import SafetyChecker
 from .args import Args
 from .globals import Globals

invokeai/backend/model_management/__init__.py

@@ -5,6 +5,7 @@ from .convert_ckpt_to_diffusers import (
     convert_ckpt_to_diffusers,
     load_pipeline_from_original_stable_diffusion_ckpt,
 )
-from .model_manager import ModelManager
+from .model_manager import ModelManager,SDModelComponent
+