Added documentation for topk (#6861)

Author: kavyasrinet

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

@@ -485,6 +485,30 @@ def max_sequence_len(rank_table):
 
 
 def topk(input, k):
+    """
+    **topk**
+
+    This function performs the operation that selects the k entries in the input
+    vector and outputs their values and indices as vectors. Thus topk_out[j] is
+    the j-th largest entry in input, and its index is topk_indices[j]
+
+    Args:
+        input (Variable|list): The input tensor that has all the data.
+        k (int): The number of top elements that the function will pick.
+
+    Returns:
+        Variable: The variable of type array that contains the k largest entries
+                  from input.
+        Variable: The variable of type array that contains the indices of k
+                  largest entries from input.
+
+    Examples:
+        .. code-block:: python
+
+          x = fluid.layers.data(name='x', shape=[10])
+          k = 5
+          array = fluid.layers.topk(x, k)
+    """
     helper = LayerHelper('topk', **locals())
     topk_out = helper.create_tmp_variable(dtype=input.data_type)
     topk_indices = helper.create_tmp_variable(dtype='int64')