add masked_select api chinese document (#1610)

* add masked_select api chinese document

* refine, test=document_preview

doc/fluid/api_cn/layers_cn.rst

@@ -169,6 +169,7 @@ fluid.layers
     layers_cn/lstm_unit_cn.rst
     layers_cn/LSTMCell_cn.rst
     layers_cn/margin_rank_loss_cn.rst
+    layers_cn/masked_select_cn.rst
     layers_cn/matmul_cn.rst
     layers_cn/maxout_cn.rst
     layers_cn/mean_cn.rst

doc/fluid/api_cn/layers_cn/masked_select_cn.rst

+.. _cn_api_fluid_layers_masked_select:
+
+masked_select
+-------------------------------
+
+.. py:function:: paddle.fluid.layers.masked_select(input, mask)
+
+该OP将根据mask Tensor的真值选取输入Tensor元素，并返回一个一维Tensor
+
+参数：
+          - **input** （Variable）- 输入Tensor，数据类型为int32, float32, float64。
+          - **mask** （Variable）- mask Tensor， 数据类型为bool。
+
+
+返回：根据mask选择后的tensor
+
+返回类型：  Variable
+
+
+**示例代码**
+
+..  code-block:: python
+
+    import paddle.fluid as fluid
+    import numpy as np
+    mask_shape = [4,1]
+    shape = [4,4]
+    data = np.random.random(mask_shape).astype("float32")
+    input_data = np.random.randint(5,size=shape).astype("float32")
+    mask_data = data > 0.5
+
+    # print(input_data)
+    # [[0.38972723 0.36218056 0.7892614  0.50122297]
+    #  [0.14408113 0.85540855 0.30984417 0.7577004 ]
+    #  [0.97263193 0.5248062  0.07655851 0.75549215]
+    #  [0.26214206 0.32359877 0.6314582  0.2128865 ]]
+
+    # print(mask_data)
+    # [[ True]
+    #  [ True]
+    #  [False]
+    #  [ True]]
+
+    input = fluid.data(name="input",shape=[4,4],dtype="float32")
+    mask = fluid.data(name="mask",shape=[4,1],dtype="bool")
+    result = fluid.layers.masked_select(input=input, mask=mask)
+    place = fluid.CPUPlace()
+    exe = fluid.Executor(place)
+    start = fluid.default_startup_program()
+    main = fluid.default_main_program()
+    exe.run(start)
+    masked_select_result= exe.run(main, feed={'input':input_data, 'mask':mask_data}, fetch_list=[result])
+    # print(masked_select_result)
+    # [0.38972723 0.36218056 0.7892614  0.50122297 0.14408113 0.85540855
+    #   0.30984417 0.7577004  0.26214206 0.32359877 0.6314582  0.2128865 ]
+
+