_query_unimplemented
BaseRetriever instance
afterwards instead of this since the former takes care of running the
registered hooks while the latter silently ignores them.
_process_unimplemented
Unstructured IO,then stored internally. This method must be
called before executing queries with the retriever.
Should be overridden by all subclasses.
.. note::
Although the recipe for forward pass needs to be defined within
this function, one should call the :class:BaseRetriever instance
afterwards instead of this since the former takes care of running the
registered hooks while the latter silently ignores them.