fix: fix calculating power law r2

Author: ChenZiHong-Gavin

graphgen/models/evaluator/kg/structure_evaluator.py

@@ -97,8 +97,8 @@ def _calculate_powerlaw_r2(degree_map: Dict[str, int]) -> Optional[float]:
             log_frequencies = np.log(frequencies)
 
             # Linear regression on log-log scale
-            r_value, *_ = stats.linregress(log_degrees, log_frequencies)
-            r2 = r_value**2
+            result = stats.linregress(log_degrees, log_frequencies)
+            r2 = result.rvalue**2
 
             return float(r2)
         except Exception as e: