Don't hardcode PGVector distance strategies (#10265)

- Description: Remove hardcoded/duplicated distance strategies in the
PGVector store.
- Issue: NA
- Dependencies: NA
- Tag maintainer: for a quicker response, tag the relevant maintainer
(see below),
- Twitter handle: @archmonkeymojo

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/vectorstores/pgvector.py

@@ -349,16 +349,16 @@ class PGVector(VectorStore):
 
     @property
     def distance_strategy(self) -> Any:
-        if self._distance_strategy == "l2":
+        if self._distance_strategy == DistanceStrategy.EUCLIDEAN:
             return self.EmbeddingStore.embedding.l2_distance
-        elif self._distance_strategy == "cosine":
+        elif self._distance_strategy == DistanceStrategy.COSINE:
             return self.EmbeddingStore.embedding.cosine_distance
-        elif self._distance_strategy == "inner":
+        elif self._distance_strategy == DistanceStrategy.MAX_INNER_PRODUCT:
             return self.EmbeddingStore.embedding.max_inner_product
         else:
             raise ValueError(
                 f"Got unexpected value for distance: {self._distance_strategy}. "
-                f"Should be one of `l2`, `cosine`, `inner`."
+                f"Should be one of {', '.join([ds.value for ds in DistanceStrategy])}."
             )
 
     def similarity_search_with_score_by_vector(