Fix pnpair_evaluator.

1. Append info into input after label and before weight.

python/paddle/trainer_config_helpers/evaluators.py

@@ -297,7 +297,8 @@ def auc_evaluator(
 def pnpair_evaluator(
         input,
         label,
-        weight,
+        info,
+        weight=None,
         name=None, ):
     """
     Positive-negative pair rate Evaluator which adapts to rank task like
@@ -307,22 +308,29 @@ def pnpair_evaluator(
 
     .. code-block:: python
 
-       eval = pnpair_evaluator(input, label, weight)
+       eval = pnpair_evaluator(input, label, info)
 
     :param input: Input Layer name. The output prediction of network.
     :type input: LayerOutput
     :param label: Label layer name.
     :type label: LayerOutput
+    :param info: Info layer name. (TODO, explaination)
+    :type info: LayerOutput
     :param weight: Weight Layer name. It should be a matrix with size
                   [sample_num, 1]. (TODO, explaination)
     :type weight: LayerOutput
     :param name: Evaluator name.
     :type name: None|basestring
     """
+    if not isinstance(input, list):
+        input = [input]
+    if label:
+        input.append(label)
+    if info:
+        input.append(info)
     evaluator_base(
         input=input,
         type="pnpair",
-        label=label,
         weight=weight,
         name=name, )