Issue 8081 Fix query results size bug. Other bug: pass vector_field param. (#8085)

@baskaryan
#8081 

Likely the reason why the issue occurred is that OpenSearch's default k
is 10, so it needs to be specified.

Here's a similar question about its cousin ElasticSearch

https://discuss.elastic.co/t/elasticsearch-returns-only-10-records-but-the-hit-is-507/136605

I tested this manually and also fixed the same issue in
`_default_painless_scripting_query`. In addition,
`_default_painless_scripting_query` was not passing the `vector_field`
name to a sub call, so I fixed that too.


![image](https://github.com/hwchase17/langchain/assets/32244272/cfb7aad1-f701-49d9-9beb-a723aa276817)

I also tested this in the aws opensearch developer tools.


![image](https://github.com/hwchase17/langchain/assets/32244272/24544682-1578-4bbb-9eb5-980463c5b41b)

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

libs/langchain/langchain/vectorstores/opensearch_vector_search.py

@@ -235,6 +235,7 @@ def _approximate_search_query_with_efficient_filter(
 
 def _default_script_query(
     query_vector: List[float],
+    k: int = 4,
     space_type: str = "l2",
     pre_filter: Optional[Dict] = None,
     vector_field: str = "vector_field",
@@ -245,6 +246,7 @@ def _default_script_query(
         pre_filter = MATCH_ALL_QUERY
 
     return {
+        "size": k,
         "query": {
             "script_score": {
                 "query": pre_filter,
@@ -258,7 +260,7 @@ def _default_script_query(
                     },
                 },
             }
-        }
+        },
     }
 
 
@@ -283,6 +285,7 @@ def __get_painless_scripting_source(
 
 def _default_painless_scripting_query(
     query_vector: List[float],
+    k: int = 4,
     space_type: str = "l2Squared",
     pre_filter: Optional[Dict] = None,
     vector_field: str = "vector_field",
@@ -292,8 +295,11 @@ def _default_painless_scripting_query(
     if not pre_filter:
         pre_filter = MATCH_ALL_QUERY
 
-    source = __get_painless_scripting_source(space_type, query_vector)
+    source = __get_painless_scripting_source(
+        space_type, query_vector, vector_field=vector_field
+    )
     return {
+        "size": k,
         "query": {
             "script_score": {
                 "query": pre_filter,
@@ -305,7 +311,7 @@ def _default_painless_scripting_query(
                     },
                 },
             }
-        }
+        },
     }
 
 
@@ -593,20 +599,20 @@ class OpenSearchVectorSearch(VectorStore):
             space_type = _get_kwargs_value(kwargs, "space_type", "l2")
             pre_filter = _get_kwargs_value(kwargs, "pre_filter", MATCH_ALL_QUERY)
             search_query = _default_script_query(
-                embedding, space_type, pre_filter, vector_field
+                embedding, k, space_type, pre_filter, vector_field
             )
         elif search_type == PAINLESS_SCRIPTING_SEARCH:
             space_type = _get_kwargs_value(kwargs, "space_type", "l2Squared")
             pre_filter = _get_kwargs_value(kwargs, "pre_filter", MATCH_ALL_QUERY)
             search_query = _default_painless_scripting_query(
-                embedding, space_type, pre_filter, vector_field
+                embedding, k, space_type, pre_filter, vector_field
             )
         else:
             raise ValueError("Invalid `search_type` provided as an argument")
 
         response = self.client.search(index=self.index_name, body=search_query)
 
-        return [hit for hit in response["hits"]["hits"][:k]]
+        return [hit for hit in response["hits"]["hits"]]
 
     def max_marginal_relevance_search(
         self,