fix: store the iteration index consistently

Fixes #1893

Coding-Agent: Codex

Codex-Version: codex-cli 0.149.0

Model: gpt-5.6-sol

Reasoning-Effort: xhigh

Author: njzjz-bot

dpgen/tools/auto_gen_param.py

@@ -55,11 +55,11 @@ class Iteration:
 
     @property
     def index_iteration(self):
-        return self._index_iteration  # pylint: disable=no-member
+        return self._index_iteration
 
     @index_iteration.setter
     def index_iteration(self, value):
-        self._index_sub_iteration = value
+        self._index_iteration = value
 
     @classmethod
     def register_iteration(cls):

tests/tools/test_auto_gen_param.py

@@ -0,0 +1,17 @@
+import unittest
+
+from dpgen.tools.auto_gen_param import Iteration
+
+
+class TestIterationIndex(unittest.TestCase):
+    def test_index_iteration_round_trip(self):
+        iteration = Iteration([50])
+        registered_index = iteration.index_iteration
+
+        self.assertIsInstance(registered_index, int)
+        iteration.index_iteration = 7
+        self.assertEqual(iteration.index_iteration, 7)
+
+
+if __name__ == "__main__":
+    unittest.main()