removed best.improvement from is_children_impurity_proxy condition

Author: gdaiha

econml/tree/_splitter.pyx

@@ -630,8 +630,8 @@ cdef class BestSplitter(Splitter):
                     best.impurity_left_val = best.impurity_left
                     best.impurity_right_val = best.impurity_right
 
-                best.improvement = self.criterion.impurity_improvement(impurity, 
-                best.impurity_left, best.impurity_right)                    
+            best.improvement = self.criterion.impurity_improvement(impurity, 
+            best.impurity_left, best.impurity_right)                    
 
         # Respect invariant for constant features: the original order of
         # element in features[:n_known_constants] must be preserved for sibling