revert CompositeMetric::add_metric

Author: jacquesqiao

python/paddle/fluid/metrics.py

@@ -116,7 +116,7 @@ def __init__(self, name=None, **kwargs):
         super(CompositeMetric, self).__init__(name, kwargs)
         self._metrics = []
 
-    def update(self, metric):
+    def add_metric(self, metric):
         if not isinstance(metric, MetricBase):
             raise ValueError("SubMetric should be inherit from MetricBase.")
         self._metrics.append(metric)