* There is the files "non_zero.cpp & non_zero.hpp" for which operator non_zero is defined"
The two files were added: non_zero.cpp & non_zero.hpp

* The pull request present a study of extension of the NonZero operatos used for ML models:
1) The various type of files was modified: cpp & hpp, onnx files & ng files
2) Non_zero operator supports the onnx standard (there are added the files in which the operator is enabled: non_zero.cpp & non_zero.hpp)
i

* Operator description: refactor

* GPU emitter - is not implemented

* GPU emitter: v0 version

* NoN zero: the input and output is adjusted to output shape & input_shape

* Move the NonZero op to v3 namespace

* Move NonZero to opset3

* Correct shape inference for NonZero

* NonZero op constant folding for 0D and 1D inputs

* Constant folding for NonZero op

* Correct output shape for NonZero & scalars

* Helper function to test NonZero

* NonZero constant folding UT for floats

* Enable more data types in NonZero CF

* NonZero type prop tests

* NonZero constant folding tests (directly)

* Use is_constant instead of casting in UT

* NonZero op doxygen docs

* onnx_importer docs adjustment

* Correct version of the NonZero core op

* Disable NonZero in GPU backend

* Short circuit if all elems in data are identical

* find_indices() optimization

* Assert on the input shape in NonZeroElements

* CF of NonZero with all non-zero values

* NonZero CF test for scalars

* bool support in NonZero

* Missing include in NonZero CF

* Dont throw if NonZero CF fails

* Update src/ngraph/pass/constant_folding_non_zero.cpp
Co-Authored-By: Robert Kimball <robert.kimball@intel.com>

* Removing warning
Co-authored-by: tomdol <tomasz.dolbniak@intel.com>
Co-authored-by: Scott Cyphers <diyessi@users.noreply.github.com>
Co-authored-by: Robert Kimball <robert.kimball@intel.com>