* Move split utility functions into core builder.

* Move activation functions to nGraph core.

* RNN cell base class.

* LSTM cell fused operator.

* Update LSTM ONNX operator to use LSTMCell fused op.

* Use Constant::create instead of make_constant.

* Remove ngraph:: prefixes and include standard headers.

* Store member shared_ptrs as object.

* Formatting.

* Run validation at the end of constructor.

* Add more doc to ActivationFunction.

* Run FusedOpDecomposition pass two times in interpreter backend.

* Remove unnecesary class member.

* Add node validation.

* Disambiguate constructors.

* Add type property test.

* Formatting and add comment with equations.

* Update IGPU backend with LSTMCell fused op.

* Fix: clip activation function input.

* Unit tests.

* Workaround for nested fused op: run FusedOpDecomposition twice.

* Fix compilation on CentOS and on GPU.

* PR feedback.

* Fix CentOS bugs.

* Address review comments.

Remove stored inputs as class members. Use node inputs directly in
decomposition.

* Fix errors.

* Review feedback: don't use decompose_op while generating Function in UTs.

* Fix merge artifacts.

* Move RNNCellBase to op/util directory.

* Fix typo for avg_pool setter method.

* Set default values for optional inputs.

* Fix typo in comment.