refine doc for metrics

hapi/metrics.py

@@ -33,10 +33,78 @@ class Metric(object):
     Base class for metric, encapsulates metric logic and APIs
 
     Usage:
-    m = SomeMetric()
-    for prediction, label in ...:
-        m.update(prediction, label)
-    m.accumulate()
+        .. code-block:: python
+
+            m = SomeMetric()
+            for prediction, label in ...:
+                m.update(prediction, label)
+            m.accumulate()
+
+    Advanced usage for :code:`add_metric_op`
+
+    Metric calculating con be accelerate by calucateing metric states
+    from model outputs and labels by Paddle OPs in :code:`add_metric_op`,
+    metric states will be fetch as numpy array and call :code:`update`
+    with states in numpy format.
+
+    Metric calculated as follows (operations in Model and Metric are
+    indicated with curly brackets, while data nodes not):
+
+                 inputs & labels              ||
+                       |                      ||
+                    {model}                   ||
+                       |                      ||
+                 outputs labels               ||
+                       |                      ||
+            {Metric.add_metric_op}            ||
+                       |                      ||
+                  metric states               ||
+                       |                      ||
+                {fetch as numpy}              ||
+                       |                      ||
+              metric states(numpy)            ||
+                       |                      ||
+                {Metric.update}               \/
+
+    Examples:
+        
+        For :code:`Accuracy` metric, which takes :code:`pred` and :code:`label`
+        as inputs, we can calculate the correct prediction matrix between
+        :code:`pred` and :code:`label` in :code:`add_metric_op`.
+
+        For examples, prediction results contains 10 classes, while :code:`pred`
+        shape is [N, 10], :code:`label` shape is [N, 1], N is mini-batch size,
+        and we only need to calculate accurary of top-1 and top-5, we could
+        calculated the correct prediction matrix of the top-5 scores of the
+        prediction of each sample like follows, while the correct prediction
+        matrix shape is [N, 5].
+
+        .. code-block:: python
+
+            def add_metric_op(pred, label):
+                # sort prediction and slice the top-5 scores
+                pred = fluid.layers.argsort(pred, descending=True)[1][:, :5]
+                # calculate whether the predictions are correct
+                correct = pred == label
+                return fluid.layers.cast(correct, dtype='float32')
+
+        With the :code:`add_metric_op`, we split some calculations to OPs(which
+        may run on GPU devices, will be faster), and only fetch 1 tensor with
+        shape as [N, 5] instead of 2 tensors with shapes as [N, 10] and [N, 1].
+        :code:`update` can be define as follows:
+
+        .. code-block:: python
+
+            def update(self, correct):
+                accs = []
+                for i, k in enumerate(self.topk):
+                    num_corrects = correct[:, :k].sum()
+                    num_samples = len(correct)
+                    accs.append(float(num_corrects) / num_samples)
+                    self.total[i] += num_corrects
+                    self.count[i] += num_samples
+                return accs
+
     """
 
     @abc.abstractmethod