Remove core safetensors->diffusers conversion models

- No longer install core conversion models. Use the HuggingFace cache to load
  them if and when needed.

- Call directly into the diffusers library to perform conversions with only shallow
   wrappers around them to massage arguments, etc.

- At root configuration time, do not create all the possible model subdirectories,
  but let them be created and populated at model install time.

- Remove checks for missing core conversion files, since they are no
  longer installed.

invokeai/backend/model_manager/probe.py

@@ -319,7 +319,7 @@ class ModelProbe(object):
     @classmethod
     def _scan_and_load_checkpoint(cls, model_path: Path) -> CkptType:
         with SilenceWarnings():
-            if model_path.suffix.endswith((".ckpt", ".pt", ".bin")):
+            if model_path.suffix.endswith((".ckpt", ".pt", ".pth", ".bin")):
                 cls._scan_model(model_path.name, model_path)
                 model = torch.load(model_path)
                 assert isinstance(model, dict)