Enable saving in SentenceTransformers by adding get_config_dict

Add `get_config_dict` method in custom_st `Transformer` for usage in SentenceTransformers that is used in the `save` method.


This fixes a bug that calling `.save` after loading with sentence-transformers 3.1 raised an `AttributeError` so now the following code successfully saves the model to local directory `jina-embeddings-v3`:

```
from sentence_transformers import SentenceTransformer
model = SentenceTransformer("jinaai/jina-embeddings-v3", trust_remote_code=True)
model.save("jina-embeddings-v3")
```

custom_st.py

@@ -160,6 +160,9 @@ class Transformer(nn.Module):
         )
         return output
 
+    def get_config_dict(self) -> dict[str, Any]:
+         return {key: self.__dict__[key] for key in self.config_keys}
+
     def save(self, output_path: str, safe_serialization: bool = True) -> None:
         self.auto_model.save_pretrained(output_path, safe_serialization=safe_serialization)
         self.tokenizer.save_pretrained(output_path)