[MO] Implement TensorFlow 2 While and Keras RNN support in MO (#3573)

* [MO] Implement TensorFlow 2 While support in MO

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Add extractors for both While and StatelessWhile and do minor changes

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Improve update_body_graph function and manage graph names properly

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Fix a map for original name of parameters from body and cond

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Implement draft version of support of TF2 Keras RNN

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Implement Keras LSTM and GRU support in MO

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Improve code for Keras RNN support

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Finalize implementation of TF2 Keras RNN support in MO

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Apply the first part of the comments after review #1

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Avoid use of explicit values of port indices in the transformation

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Finalize code after the first-round review

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

* Apply comments after the second-round review

Signed-off-by: Roman Kazantsev <roman.kazantsev@intel.com>

model-optimizer/extensions/back/SpecialNodesFinalization.py

@@ -1,5 +1,5 @@
 """
- Copyright (C) 2018-2020 Intel Corporation
+ Copyright (C) 2018-2021 Intel Corporation
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
@@ -15,7 +15,6 @@
 """
 import logging as log
 from collections import defaultdict
-from copy import copy
 
 import numpy as np
 
@@ -125,6 +124,11 @@ class RemoveConstToResult(BackReplacementPattern):
     """
     enabled = True
     force_clean_up = True
+    # TODO: remove this transformation once all plugins support constant value network.
+    # Do not run recursively since Const->Result sub-graph can be encountered in a body graph of Loop node
+    # and this sub-graph is needed to avoid dynamism created by Loop node
+    # in case using axis in output port map
+    run_not_recursively = True
 
     @staticmethod
     def pattern():