Config: Use an explicit "auto" value for rope_alpha

Using "auto" for rope alpha removes ambiguity on how to explicitly
enable automatic rope calculation. The same behavior of None -> auto
calculate still exists, but can be overwritten if a model's tabby_config.yml
includes `rope_alpha`.

Signed-off-by: kingbri <bdashore3@proton.me>

common/model.py

@@ -149,6 +149,7 @@ async def unload_embedding_model():
     embeddings_container = None
 
 
+# FIXME: Maybe make this a one-time function instead of a dynamic default
 def get_config_default(key: str, model_type: str = "model"):
     """Fetches a default value from model config if allowed by the user."""