Add doc for CPUPlace CUDAPlace CUDAPinPlace (#16442)

test=develop

paddle/fluid/pybind/pybind.cc

@@ -769,7 +769,11 @@ All parameter, weight, gradient are variables in Paddle.
 #if (defined(PADDLE_WITH_CUDA) && !defined(_WIN32))
   py::class_<platform::Communicator>(m, "Communicator").def(py::init<>());
 #endif
-  py::class_<platform::CUDAPlace>(m, "CUDAPlace")
+  py::class_<platform::CUDAPlace>(m, "CUDAPlace", R"DOC(
+    CUDAPlace is a descriptor of a device. It represents a GPU, and each CUDAPlace
+    has a dev_id to indicate the number of cards represented by the current CUDAPlace.
+    The memory of CUDAPlace with different dev_id is not accessible.
+        )DOC")
       .def("__init__",
            [](platform::CUDAPlace &self, int dev_id) {
 #ifdef PADDLE_WITH_CUDA
@@ -790,7 +794,10 @@ All parameter, weight, gradient are variables in Paddle.
            &IsSamePlace<platform::CUDAPlace, platform::CUDAPinnedPlace>)
       .def("__str__", string::to_string<const platform::CUDAPlace &>);
 
-  py::class_<paddle::platform::CPUPlace>(m, "CPUPlace")
+  py::class_<paddle::platform::CPUPlace>(m, "CPUPlace", R"DOC(
+    CPUPlace is a descriptor of a device. It represents a CPU, and the memory
+    CPUPlace can be accessed by CPU.
+        )DOC")
       .def(py::init<>())
       .def("_type", &PlaceIndex<platform::CPUPlace>)
       .def("_equals", &IsSamePlace<platform::CPUPlace, platform::Place>)
@@ -800,7 +807,10 @@ All parameter, weight, gradient are variables in Paddle.
            &IsSamePlace<platform::CPUPlace, platform::CUDAPinnedPlace>)
       .def("__str__", string::to_string<const platform::CPUPlace &>);
 
-  py::class_<paddle::platform::CUDAPinnedPlace>(m, "CUDAPinnedPlace")
+  py::class_<paddle::platform::CUDAPinnedPlace>(m, "CUDAPinnedPlace", R"DOC(
+    CUDAPinnedPlace is a descriptor of a device. The memory of CUDAPinnedPlace
+    can be accessed by GPU and CPU.
+        )DOC")
       .def("__init__",
            [](platform::CUDAPinnedPlace &self) {
 #ifndef PADDLE_WITH_CUDA