Add `Model` api cn docs (#2131)

* add hapi Model, test=develop

* fix save inference model example, test=develop

* fix sample code, test=develop

* fix import, test=develop

* fix import, test=develop

* fix sample code, test=develop

* fix sample code, test=develop

* fix comments, test=develop

doc/fluid/api_cn/incubate_cn.rst

+=======================
+paddle.incubate
+=======================
+
+
+
+
+..  toctree::
+    :maxdepth: 1
+
+    incubate_cn/hapi_cn.rst

doc/fluid/api_cn/incubate_cn/hapi_cn.rst

+=======================
+hapi
+=======================
+
+
+
+
+..  toctree::
+    :maxdepth: 1
+
+    hapi_cn/Model_cn.rst
+    hapi_cn/set_device_cn.rst

doc/fluid/api_cn/index_cn.rst

@@ -7,6 +7,7 @@ API Reference
 
     ../api_guides/index_cn.rst
     framework_cn.rst
+    incubate_cn.rst
     nn_cn.rst
     tensor_cn.rst
     fluid_cn.rst