[ONNX] Add verification of split attribute in split op (#2669)
[ONNX] Add verification of split attribute in split op (#2669)

* [ONNX] Add verification of split attribute in split op

* Style

* Update split.cpp

* Add verification of split length

NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph