[Frontend] pickle.dumps support for FrontEndManager (#6370)

* [Frontend] pickle.dumps support for FrontEndManager

* Fix clang-format

* Reorganize imports in test_frontendmanager.py

ngraph/python/src/pyngraph/frontend/frontend_manager.cpp

@@ -21,6 +21,10 @@ void regclass_pyngraph_FrontEndManager(py::module m)
 
     fem.def(py::init<>());
 
+    // Empty pickle dumps are supported as FrontEndManager doesn't have any state
+    fem.def(py::pickle([](const ngraph::frontend::FrontEndManager&) { return py::make_tuple(0); },
+                       [](py::tuple t) { return ngraph::frontend::FrontEndManager(); }));
+
     fem.def("get_available_front_ends",
             &ngraph::frontend::FrontEndManager::get_available_front_ends,
             R"(

ngraph/python/tests/test_ngraph/test_frontendmanager.py

@@ -1,13 +1,16 @@
 # Copyright (C) 2018-2021 Intel Corporation
 # SPDX-License-Identifier: Apache-2.0
 
-import numpy as np
-import pytest
+import pickle
 
 from ngraph import PartialShape
 from ngraph.frontend import FrontEndCapabilities, FrontEndManager, InitializationFailure
 from ngraph.utils.types import get_element_type
 
+import numpy as np
+
+import pytest
+
 mock_available = True
 try:
     from pybind_mock_frontend import get_fe_stat, get_mdl_stat, get_place_stat
@@ -24,6 +27,10 @@ mock_needed = pytest.mark.skipif(not mock_available,
 
 
 # ---------- FrontEnd tests ---------------
+def test_pickle():
+    pickle.dumps(fem)
+
+
 @mock_needed
 def test_load_by_framework_caps():
     frontEnds = fem.get_available_front_ends()