Fix LogOdds.calibrate to compute the mean once instead of per score (#1365)

src/python/txtai/embeddings/search/hybrid.py

@@ -194,7 +194,8 @@ class LogOdds:
         array = [s for s in raw if s > 0]
         if array:
             median = sorted(array)[len(array) // 2]
-            std = (sum((x - sum(array) / len(array)) ** 2 for x in array) / len(array)) ** 0.5
+            mean = sum(array) / len(array)
+            std = (sum((x - mean) ** 2 for x in array) / len(array)) ** 0.5
             alpha = 1.0 / std if std > 0 else 1.0
 
         return median, alpha