* Helper function converting ONNX auto_pad into nGraph PadType.

Separate auto_pad support from explicitly provided paddings.

* Add support for more PadType values for GroupConvolutionTranspose.

* Pass auto_pad attribute value to nGraph operator.

* Helper class for generating ONNX pooling operators.

* Pass auto pad type to nGraph Convolution operator.

* Use pooling factory.

* Helper function calculating pads taking into account auto_pad attribute.

* Fix attribute type in UT ONNX models.

* Take auto_pad attribute value into account.

* Rename helper function and update doc.

* Retain old API for GroupConvoloutionTranspose

* Remove PadType::INVALID and use other approach to validate auto_pad
value.

* Style apply.

* Fix clang err on documentation style.

* Refactor get_auto_pad method.

* Fix segfaults on CentOS 7.

When using const reference the node member m_attributes had invalid
data.