Max marginal relecance search fails if there are not enough docs (#1117)

Implementation fails if there are not enough documents. Added the same check as used for similarity search. Current implementation raises ``` File ".venv/lib/python3.9/site-packages/langchain/vectorstores/faiss.py", line 160, in max_marginal_relevance_search _id = self.index_to_docstore_id[i] KeyError: -1 ```

Author: tmbo

langchain/vectorstores/faiss.py

@@ -189,6 +189,9 @@ def max_marginal_relevance_search_by_vector(
  docs = []
  for i in selected_indices:
  _id = self.index_to_docstore_id[i]
+ if _id == -1:
+ # This happens when not enough docs are returned.
+ continue
  doc = self.docstore.search(_id)
  if not isinstance(doc, Document):
  raise ValueError(f"Could not find document for id {_id}, got {doc}")