Update lambdamart.py

Solve the overflow caused by  very small sumd.

pyltr/models/lambdamart.py

@@ -315,7 +315,7 @@ def _update_terminal_regions(self, tree, X, y, lambdas, deltas, y_pred,
             terminal_region = np.where(masked_terminal_regions == leaf)
             suml = np.sum(lambdas[terminal_region])
             sumd = np.sum(deltas[terminal_region])
-            tree.value[leaf, 0, 0] = 0.0 if sumd == 0.0 else (suml / sumd)
+            tree.value[leaf, 0, 0] = 0.0 if abs(sumd) < 1e-300 else (suml / sumd)
 
         y_pred += tree.value[terminal_regions, 0, 0] * self.learning_rate