Remove convert_to_tensor=True in MultiVectorEncoder, this parameter was removed

Hello!

This simple PR removes `convert_to_tensor=True` in the MultiVectorEncoder `encode_query`/`encode_document`/`encode` calls, as I removed this parameter and made a Tensor output the default just prior to the release. I didn't realize these snippets still used this parameter, my apologies!

- Tom Aarsen

README.md

@@ -83,8 +83,8 @@ images = [
     "https://huggingface.co/datasets/sentence-transformers/example-documents/resolve/main/doc4.jpg",
 ]
 
-query_embeddings = model.encode_query(queries, convert_to_tensor=True)
-document_embeddings = model.encode_document(images, convert_to_tensor=True)
+query_embeddings = model.encode_query(queries)
+document_embeddings = model.encode_document(images)
 print(f"Query 0 shape:    {tuple(query_embeddings[0].shape)}")
 print(f"Document 0 shape: {tuple(document_embeddings[0].shape)}")
 # Query 0 shape:    (23, 128)