* onnx: add 'constant' operator
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: getting attribute value by name
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: fix code style
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: fix clang compilation warnings
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>

* onnx: incorporate review comments
Signed-off-by: Artur Wojcik <artur.wojcik@intel.com>