Add `save_in_root` to the custom Module to allow saving & loading locally

Hello!

## Pull Request overview
* Add `save_in_root` to the custom Module to allow saving & loading locally

## Details
Just like in [jina-embeddings-v3](https://huggingface.co/jinaai/jina-embeddings-v3/blob/main/custom_st.py#L36), we can add `save_in_root = True` to ensure that this module is saved in the root of the directory. This is required because `transformers` expects any custom modeling files (such as `custom_st.py` itself) to be in the root. Without this change, we'll get this after saving with Sentence Transformers:


With this change in place, we'll instead get this:


and then you'll be able to save and load this correctly in Sentence Transformers.

- Tom Aarsen

custom_st.py

@@ -12,6 +12,9 @@ from transformers import AutoConfig, AutoImageProcessor, AutoModel, AutoTokenize
 
 
 class Transformer(nn.Module):
+
+    save_in_root: bool = True
+    
     def __init__(
         self,
         model_name_or_path: str = 'jinaai/jina-clip-v2',