convert custom VAEs into diffusers

- When a legacy checkpoint model is loaded via --convert_ckpt and its
  models.yaml stanza refers to a custom VAE path (using the 'vae:'
  key), the custom VAE will be converted and used within the diffusers
  model. Otherwise the VAE contained within the legacy model will be
  used.

- Note that the heuristic_import() method, which imports arbitrary
  legacy files on disk and URLs, will continue to default to the
  the standard stabilityai/sd-vae-ft-mse VAE. This can be fixed after
  the fact by editing the models.yaml stanza using the Web or CLI
  UIs.

- Fixes issue #2917

invokeai/frontend/CLI/CLI.py

@@ -772,16 +772,10 @@ def convert_model(model_name_or_path: Union[Path, str], gen, opt, completer):
             original_config_file = Path(model_info["config"])
             model_name = model_name_or_path
             model_description = model_info["description"]
-            vae = model_info["vae"]
+            vae_path = model_info.get("vae")
         else:
             print(f"** {model_name_or_path} is not a legacy .ckpt weights file")
             return
-        if vae_repo := invokeai.backend.model_management.model_manager.VAE_TO_REPO_ID.get(
-            Path(vae).stem
-        ):
-            vae_repo = dict(repo_id=vae_repo)
-        else:
-            vae_repo = None
         model_name = manager.convert_and_import(
             ckpt_path,
             diffusers_path=Path(
@@ -790,7 +784,7 @@ def convert_model(model_name_or_path: Union[Path, str], gen, opt, completer):
             model_name=model_name,
             model_description=model_description,
             original_config_file=original_config_file,
-            vae=vae_repo,
+            vae_path=vae_path,
         )
     else:
         try: