Adding doc for sums layer (#6857)

python/paddle/v2/fluid/layers/tensor.py

@@ -56,9 +56,28 @@ def concat(input, axis=0):
 
 
 def sums(input, out=None):
-    """
-    This function takes in the input and performs the sum operation on it
-    and returns that as the output.
+    """This function performs the sum operation on the input and returns the
+    result as the output.
+
+    Args:
+        input (Variable|list): The input tensor that has the elements
+                               that need to be summed up.
+
+    Returns:
+        Variable: The tensor type variable that has the sum of input
+                  written to it.
+
+    Examples:
+        .. code-block::python
+
+          tmp = fluid.layers.zeros(shape=[10], dtype='int32')
+          i = fluid.layers.fill_constant(shape=[1], dtype='int64', value=10)
+          a0 = layers.array_read(array=tmp, i=i)
+          i = layers.increment(x=i)
+          a1 = layers.array_read(array=tmp, i=i)
+          mean_a0 = layers.mean(x=a0)
+          mean_a1 = layers.mean(x=a1)
+          a_sum = layers.sums(input=[mean_a0, mean_a1])
     """
     helper = LayerHelper('sum', **locals())
     if out is None: