* inserted Convert to unsigned
* moved declarations from hpp into cpp, specification corrected
* added static const modifier
* updated convert specification
* minor corrections
* split into 3 passes(Init, Propogate, Update), renamed final pass to ConvertNmsGatherPathToUnsigned
* added description why transformation is needed
* added matcher for several NMS versions, removed TRANSFORMATIONS_API macros from cpp
* applied comments:
- used GraphRewrite instead of FunctionPass
- simplified some expressions
- corrected case when Converts output goes to multiple nodes
- added to MOC transformations
- other minor corrections
* removed redundant namespace prefixes
* fixed #include <ngraph/pass/graph_rewrite.hpp>
* removed matcher_scope, debug code, and redundant dynamic_cast
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