[ONNX] Fix support for float16 in ONNX frontend. (#3563)

Initial support for float16 was done before the ngraph::float16 data
type was introduced in nGraph. This PR aligns ONNX frontend with the
latest float16 implementation.

src/ngraph/frontend/onnx_import/op/constant.cpp

@@ -48,7 +48,7 @@ namespace ngraph
                     inline std::shared_ptr<ngraph::op::Constant>
                         make_ng_constant<Tensor::Type::float16>(const Tensor& tensor)
                     {
-                        return __make_ng_constant<float>(element::f32, tensor);
+                        return __make_ng_constant<ngraph::float16>(element::f16, tensor);
                     }
 
                     template <>