Get instance of an existing PGVector store.This method will return the instance of the store without inserting any new embeddings
afrom_existing_index(
cls: Type[PGVector],
embedding: Embeddings,
*,
collection_name: str = _LANGCHAIN_DEFAULT_COLLECTION_NAME,
distance_strategy: DistanceStrategy = DEFAULT_DISTANCE_STRATEGY,
pre_delete_collection: bool = False,
connection: Optional[DBConnection] = None,
**kwargs: Any = {}
) -> PGVector