rm fluid.core in desigin_idea (#674)

* Update fluid_design_idea.md

* Update fluid_design_idea_en.md

doc/fluid/advanced_usage/design_idea/fluid_design_idea.md

@@ -194,7 +194,7 @@ Fluid中使用fluid.Executor(place)创建Executor，place属性由用户定义
 下例代码表示创建一个Executor，其运行场所在CPU内：
 
 ```python
-cpu=core.CPUPlace()
+cpu=fluid.CPUPlace()
 exe = fluid.Executor(cpu)
 ```
 
@@ -329,7 +329,7 @@ Fluid使用Executor来执行网络训练，Executor运行细节请参考[Executo
 创建Executor只需调用 fluid.Executor(place) 即可，在此之前请您依据训练场所定义place变量：
 ```python
  #在CPU内执行训练
- cpu = fluid.core.CPUPlace()
+ cpu = fluid.CPUPlace()
  #创建Executor
  exe = fluid.Executor(cpu)
 ```

doc/fluid/advanced_usage/design_idea/fluid_design_idea_en.md

@@ -195,7 +195,7 @@ Fluid uses Fluid.Executor(place) to create an Executor. The place attribute is d
 The following code example creates an Executor that runs on CPU:
 
 ```python
-cpu=core.CPUPlace()
+cpu=fluid.CPUPlace()
 exe = fluid.Executor(cpu)
 ```
 
@@ -330,7 +330,7 @@ Fluid uses Executor to perform network training. For details on Executor operati
 To create an Executor, simply call fluid.Executor(place). Before that, please define a place variable based on the training site:
 ```python
  #Execute training on CPU
- cpu = fluid.core.CPUPlace()
+ cpu = fluid.CPUPlace()
  #Create Executor
  exe = fluid.Executor(cpu)
 ```