executor_en (#601)

* executor_en

* Review

* file path+review x2

doc/fluid/api_guides/low_level/layers/executor_en.rst

+..  _api_guide_executor_en:
+
+################
+Executor
+################
+
+:code:`Executor` realizes a simple executor in which all operators will be executed in order. You can run :code:`Executor` in a Python script. There are two kinds of executors in PaddlePaddle Fluid. One is single-thread executor which is the default option for :code:`Executor` 
+and another is the parallel executor which is illustrated in :ref:`api_guide_parallel_executor_en` .
+
+The logic of :code:`Executor` is very simple. It is suggested to thoroughly run the model with :code:`Executor` in debugging phase on one computer and then switch to mode of multiple devices or multiple computers to compute.
+
+:code:`Executor` receives a :code:`Place` at construction, which can either be :ref:`api_fluid_CPUPlace` or :ref:`api_fluid_CUDAPlace`. 
+
+For simple example please refer to `quick_start_fit_a_line <http://paddlepaddle.org/documentation/docs/zh/1.1/beginners_guide/quick_start/fit_a_line/README.html>`_ 
+
+For API Reference, please refer to :ref:`api_fluid_Executor`.
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