Add doc for while op

Author: panyx0718

python/paddle/fluid/layers/control_flow.py

@@ -607,6 +607,60 @@ def __exit__(self, exc_type, exc_val, exc_tb):
 
 
 class While(object):
+    """
+    while loop control flow.
+
+    Args:
+        cond (Variable): condition used to compare.
+        name (str): The name of this layer.
+
+    Examples:
+          .. code-block:: python
+
+            # The value these d0, d1 and d2 can be fed from python.
+            d0 = fluid.layers.data(
+                "d0", shape=[10], append_batch_size=False, dtype='float32')
+            d1 = fluid.layers.data(
+                "d1", shape=[10], append_batch_size=False, dtype='float32')
+            d2 = fluid.layers.data(
+                "d2", shape=[10], append_batch_size=False, dtype='float32')
+            i = fluid.layers.zeros(shape=[1], dtype='int64')
+            i.stop_gradient = True
+            init = fluid.layers.zeros(shape=[10], dtype='float32')
+            # Initialize mem_array from init
+            mem_array = fluid.layers.array_write(x=init, i=i)
+            # Initialize data_array from d0
+            data_array = fluid.layers.array_write(x=d0, i=i)
+            # Set a value to data_array using d1[i].
+            i = fluid.layers.increment(i)
+            fluid.layers.array_write(d1, i, array=data_array)
+            # Set a value to data_array using d2[i].
+            i = fluid.layers.increment(i)
+            fluid.layers.array_write(d2, i, array=data_array)
+            # Create a idx to start the while loop.
+            i = fluid.layers.zeros(shape=[1], dtype='int64')
+            i.stop_gradient = True
+
+            array_len = fluid.layers.fill_constant(
+                shape=[1], dtype='int64', value=3)
+            array_len.stop_gradient = True
+            # Create the while loop condition.
+            cond = fluid.layers.less_than(x=i, y=array_len)
+
+            # Within the loop, perform sums.
+            while_op = fluid.layers.While(cond=cond)
+            with while_op.block():
+                d = fluid.layers.array_read(array=data_array, i=i)
+                prev = fluid.layers.array_read(array=mem_array, i=i)
+                result = fluid.layers.sums(input=[d, prev])
+
+                i = fluid.layers.increment(x=i, in_place=True)
+                fluid.layers.array_write(result, i=i, array=mem_array)
+                fluid.layers.less_than(x=i, y=array_len, cond=cond)
+
+            sum_result = fluid.layers.array_read(array=mem_array, i=i)
+    """
+
     BEFORE_WHILE_BLOCK = 0
     IN_WHILE_BLOCK = 1
     AFTER_WHILE_BLOCK = 2