Add scope doc (#14582)

* add doc for scope

* update doc for force_init_on_cpu
test=develop

* follow comment test=develop

* update format test=develop

Author: jacquesqiao

paddle/fluid/pybind/pybind.cc

@@ -398,7 +398,26 @@ All parameter, weight, gradient are variables in Paddle.
             },
         py::return_value_policy::copy);
 
-  py::class_<Scope>(m, "Scope", "")
+  py::class_<Scope>(m, "Scope", R"DOC(
+    Scope is an association of a name to Variable. All variables belong to Scope.
+
+    Variables in a parent scope can be retrieved from local scope.
+
+    You need to specify a scope to run a Net, i.e., `exe.Run(&scope)`.
+    One net can run in different scopes and update different variable in the
+    scope.
+
+    You can create var in a scope and get it from the scope.
+
+    Examples:
+        .. code-block:: python
+
+          # create tensor from a scope and set value to it.
+          param = scope.var('Param').get_tensor()
+          param_array = np.full((height, row_numel), 5.0).astype("float32")
+          param.set(param_array, place)
+
+        )DOC")
       .def("var",
            [](Scope &self, const std::string &name) -> Variable * {
              return self.Var(name);

python/paddle/fluid/initializer.py

@@ -33,13 +33,15 @@ def force_init_on_cpu():
     """
     The flag of whether force to init variables on CPU.
 
-    Returns::
+    Returns:
+        bool: the state if we should force init on CPU.
 
     Examples:
+
         .. code-block:: python
 
             if force_init_on_cpu():
-                pass
+                create_op('force_cpu': force_init_on_cpu())
 
     """
     return _force_init_on_cpu_