Change default MapKeyInfo (#3194)

Summary:
I want to converge around the "step" column name (as we use in the API) as opposed to "steps" -- this may not matter in the long run as we rethink mapdata, but I think now is the time to do it.

cc ltiao who has diffs in flight that may be effected by this.

Lets see if this breaks anything -- shouldnt actually be used anywhere except maybe tests.

Reviewed By: saitcakmak

Differential Revision: D67413011

ax/core/map_metric.py

@@ -30,4 +30,4 @@ class MapMetric(Metric):
     """
 
     data_constructor: type[MapData] = MapData
-    map_key_info: MapKeyInfo[float] = MapKeyInfo(key="steps", default_value=0.0)
+    map_key_info: MapKeyInfo[float] = MapKeyInfo(key="step", default_value=0.0)

ax/modelbridge/transforms/tests/test_map_key_to_float_transform.py

@@ -93,7 +93,7 @@ def test_Init(self) -> None:
                     "parameters": {MapKeyToFloat.DEFAULT_MAP_KEY: {"log_scale": False}}
                 },
             )
-            self.assertDictEqual(t.parameters, {"steps": {"log_scale": False}})
+            self.assertDictEqual(t.parameters, {"step": {"log_scale": False}})
 
             self.assertEqual(len(t._parameter_list), 1)