Add workaround for control edges to support TF 2.4 RNN (#4633)

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

model-optimizer/extensions/front/tf/while_ext.py

@@ -51,6 +51,12 @@ def update_body_graph(body_graph: Graph, subgraph_proto: dict,
         # add incoming edges based on data_nodes_map
         for dst_port, inp in enumerate(pb_node.input):
             orig_src_id = inp.split(":")[0]
+
+            # TODO: avoid this temporal workaround for TF 2.4 or higher RNN layers:
+            #  skip control flow dependency
+            if orig_src_id[0] == '^':
+                continue
+
             src_id = map_original_name[orig_src_id]
             src_port = 0 if len(inp.split(":")) == 1 else int(inp.split(":")[-1])
             assert (body_graph.has_node(src_id))