* NonMaxSuppression op skeleton

* Validation of the NonMaxSuppresion op

* Correct last 'boxes' dimention check

* onnx_importer support for NonMaxSuppression

* Code formatting

* Type and shape inference for NonMaxSuppression

* Different initialization of NMS inputs in onnx_importer

* Code formatting

* Basic type_prop tests for NonMaxSuppression

* More type_prop validation for NMS