New versions of Assign/ReadValue ops (#4055)

* new versions of assign/read value, squashed commits

* Assign/ReadValue base classes, code style

* update ie_ir_parser and conversion

* low latency transformation update to support Loop op

* ngraph code style

* fix build, refactoring

* fix unit tests

* fix unit tests again

* fix unit tests

* fix code style

* update comments

* ngraph codestyle

* fix low latency tests

* fix failed unit tests

* fix incorrect version of submodule

* fix review remarks

* ngraph code style

* fix ngraph tests

* fix low latency unit tests

model-optimizer/mo/ops/assign.py

@@ -24,7 +24,7 @@ class Assign(Op):
         super().__init__(graph, {
             'type': self.op,
             'op': self.op,
-            'version': 'opset3',
+            'version': 'opset6',
             'infer': self.infer,
             'in_ports_count': 1,
             'out_ports_count': 1,

model-optimizer/mo/ops/read_value.py

@@ -24,7 +24,7 @@ class ReadValue(Op):
         super().__init__(graph, {
             'type': self.op,
             'op': self.op,
-            'version': 'opset3',
+            'version': 'opset6',
             'infer': self.infer,
             'type_infer': self.type_infer,
             'in_ports_count': 1,