Speed up query list generation for dense matching (#453)

Author: mihaidusmanu

hloc/match_dense.py

@@ -485,7 +485,7 @@ def match_and_assign(
     pairs = parse_retrieval(pairs_path)
     pairs = [(q, r) for q, rs in pairs.items() for r in rs]
     pairs = find_unique_new_pairs(pairs, None if overwrite else match_path)
-    required_queries = set(sum(pairs, ()))
+    required_queries = set(chain.from_iterable(pairs))
 
     name2ref = {
         n: i for i, p in enumerate(feature_paths_refs) for n in list_h5_names(p)