model_save_reader_en (#600)

* model_save_reader_en

* Review

* delete redundant

doc/fluid/api_guides/low_level/model_save_reader_en.rst

+..  _api_guide_model_save_reader_en:
+
+#######################
+Save and Load a Model
+#######################
+
+To save and load model, there are eight APIs playing an important role:
+:code:`fluid.io.save_vars`, :code:`fluid.io.save_params`, :code:`fluid.io.save_persistables`, :code:`fluid.io.save_inference_model`, :code:`fluid.io.load_vars`, :code:`fluid.io.load_params`, :code:`fluid.io.load_persistables` and :code:`fluid.io.load_inference_model` .
+
+Variables, Persistables and Parameters
+================================================
+
+In :code:`Paddle` , every input and output of operator( :code:`Operator` ) is a variable( :code:`Variable` ), and parameter( :code:`Parameter` ) is a derived class of Variable( :code:`Variable` ). Persistables (:code:`Persistables`) are variables that won't be deleted after each iteration. Parameter is a kind of persistable variable which will be updated by optimizer ( :ref:`api_guide_optimizer_en` ) after each iteration. Training of neural network in essence is to update parameters.
+
+Introduction to APIs for saving a model
+========================================
+
+- :code:`fluid.io.save_vars`: Variables are saved in specified directory by executor( :ref:`api_guide_executor_en` ). There are two ways to save variables:
+
+  1）Set :code:`vars` in the API to assign the variable list to be saved.
+
+  2）Assign an existed program( :code:`Program` ) to :code:`main_program` in the API, and then all variables in the program will be saved.
+
+  The first one has a higher priority than the second one.
+
+  For API Reference , please refer to :ref:`api_fluid_io_save_vars`.
+
+- :code:`fluid.io.save_params`: Set :code:`main_program` in the API with the model Program( :code:`Program` ). This API will filter all parameters( :code:`Parameter` ) of targeted program and save them in folder assigned by :code:`dirname` or file assigned by :code:`filename` .
+
+  For API Reference , please refer to :ref:`api_fluid_io_save_params`.
+
+- :code:`fluid.io.save_persistables`: :code:`main_program` of API assigns program( :code:`Program` ). This API will filter all persistables( :code:`persistable==True` ) of targeted program and save them in folder assigned by :code:`dirname` or file assigned by :code:`filename` .
+
+  For API Reference, please refer to :ref:`api_fluid_io_save_persistables`.
+
+- :code:`fluid.io.save_inference_model`: please refer to  :ref:`api_guide_inference_en`.  
+
+Introduction to APIs for loading a model
+========================================
+
+- :code:`fluid.io.load_vars`: Executor( :code:`Executor` ) loads variables into the target directory. There are two ways to load variables:
+  
+  1）:code:`vars` in the API assigns variable list to be loaded.
+  
+  2）Assign an existed program( :code:`Program` ) to the :code:`main_program` field in the API, and then all variables in the program will be loaded.
+
+  The first loading method has higher priority than the second one.
+
+  For API Reference, please refer to :ref:`api_fluid_io_load_vars`.
+
+- :code:`fluid.io.load_params`: This API filters all parameters( :code:`Parameter` ) in program assigned by :code:`main_program` and load these parameters from folder assigned by :code:`dirname` or file assigned by :code:`filename` .
+
+  For API Reference, please refer to :ref:`api_fluid_io_load_params` .
+
+- :code:`fluid.io.load_persistables`:This API filters all persistables( :code:`persistable==True` ) in program assigned by :code:`main_program` and load these persistables from folder assigned by :code:`dirname` or file assigned by :code:`filename` .
+
+  For API Reference, please refer to :ref:`api_fluid_io_load_persistables` .
+
+-  :code:`fluid.io.load_inference_model`: please refer to :ref:`api_guide_inference_en` .
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