* Added FP32 to FP16 conversion transformation

* Update src/ngraph/pass/convert_fp32_to_fp16.cpp
Co-Authored-By: Robert Kimball <robert.kimball@intel.com>

* Update src/ngraph/pass/convert_fp32_to_fp16.hpp
Co-Authored-By: Robert Kimball <robert.kimball@intel.com>

* Added NGRAPH_API for fp32 to fp16 conversion
Co-authored-by: Scott Cyphers <diyessi@users.noreply.github.com>
Co-authored-by: Robert Kimball <robert.kimball@intel.com>