Fix several cn doc sample code diff (#1967)

* fix build strategy cn doc sample code diff

* fix code error

* fix execution strategy error

* fix LayerNorm error

doc/fluid/api_cn/dygraph_cn/LayerNorm_cn.rst

@@ -47,7 +47,7 @@ LayerNorm
     x = numpy.random.random((3, 32, 32)).astype('float32')
     with fluid.dygraph.guard():
         x = to_variable(x)
-        layernorm = fluid.LayerNorm('LayerNorm', begin_norm_axis=1)
-        ret = layernorm(x)
+        layerNorm = fluid.LayerNorm([32, 32])
+        ret = layerNorm(x)
 
 

doc/fluid/api_cn/fluid_cn/BuildStrategy_cn.rst

@@ -7,7 +7,7 @@ BuildStrategy
 
 .. py:class:: paddle.fluid.BuildStrategy
 
-``BuildStrategy`` 使用户更方便地控制[ ``ParallelExecutor`` ](../fluid_cn.html\#parallelexecutor)中计算图的建造方法，可通过设置 ``ParallelExecutor`` 中的 ``BuildStrategy`` 成员来实现此功能。
+``BuildStrategy`` 使用户更方便地控制 :ref:`cn_api_fluid_ParallelExecutor` 中计算图的建造方法，可通过设置 ``ParallelExecutor`` 中的 ``BuildStrategy`` 成员来实现此功能。
 
 **代码示例**
 
@@ -68,6 +68,7 @@ bool类型。表明是否融合(fuse) broadcast ops。该选项指在Reduce模
 **代码示例**
 
 .. code-block:: python
+
     import paddle.fluid as fluid
     build_strategy = fluid.BuildStrategy()
     build_strategy.fuse_broadcast_ops = True
@@ -108,6 +109,7 @@ bool类型。表明是否融合(fuse) relu和depthwise_conv2d，节省GPU内存
 
     import os
     import numpy as np
+    import paddle.fluid as fluid
     import paddle.fluid.compiler as compiler
 
     use_cuda = True

doc/fluid/api_cn/fluid_cn/ExecutionStrategy_cn.rst

@@ -33,7 +33,7 @@ ExecutionStrategy
 
     train_exe = fluid.ParallelExecutor(use_cuda=False,
                                        loss_name=avg_loss.name,
-                                     exec_strategy=exec_strategy)
+                                       exec_strategy=exec_strategy)
 
 
 .. py:attribute:: num_iteration_per_drop_scope