Add regression-target stds (normalize MAE -> NMAE)

regression_stds.json

@@ -0,0 +1,20 @@
+{
+  "_comment": "Per-task standard deviation (ddof=1) of the regression target on the train split, for the RelBench v1 datasets. Used to normalize MAE into NMAE (nmae = mae / std). Keyed by '<dataset>/<task>'.",
+  "stds": {
+    "rel-amazon/item-ltv": 590.9188248003885,
+    "rel-amazon/review-rating": 1.0156891345977783,
+    "rel-amazon/user-ltv": 57.505195743487945,
+    "rel-avito/ad-ctr": 0.09567494340715577,
+    "rel-event/user-attendance": 0.7651533955340627,
+    "rel-event/users-birthyear": 9.004328063087724,
+    "rel-f1/driver-position": 7.025733902396073,
+    "rel-f1/qualifying-position": 6.638883864730652,
+    "rel-f1/results-position": 4.2514188227363245,
+    "rel-hm/item-sales": 0.49532132693681274,
+    "rel-hm/transactions-price": 0.018997141122757227,
+    "rel-stack/post-votes": 0.5104313497537508,
+    "rel-trial/site-success": 0.4758632591493917,
+    "rel-trial/studies-enrollment": 191565.62428113908,
+    "rel-trial/study-adverse": 339.11053267803163
+  }
+}