[ONNX] Expose API enabling to get supported ONNX operators set. (#2202)

* Expose API enabling to get supported ONNX operators set.

* Review comments.

- Don't expose operator function handlers, just return list of names.

* Function rename.

src/ngraph/frontend/onnx_import/onnx.cpp

@@ -99,6 +99,18 @@ namespace ngraph
             OperatorsBridge::register_operator(name, version, domain, std::move(fn));
         }
 
+        std::set<std::string> get_supported_operators(std::int64_t version,
+                                                      const std::string& domain)
+        {
+            OperatorSet op_set{OperatorsBridge::get_operator_set(version, domain)};
+            std::set<std::string> op_list{};
+            for (const auto& op : op_set)
+            {
+                op_list.emplace(op.first);
+            }
+            return op_list;
+        }
+
     } // namespace onnx_import
 
 } // namespace ngraph

src/ngraph/frontend/onnx_import/onnx.hpp

@@ -16,7 +16,9 @@
 
 #pragma once
 
+#include <cstdint>
 #include <iostream>
+#include <set>
 #include <string>
 
 #include "core/operator_set.hpp"
@@ -40,6 +42,16 @@ namespace ngraph
                                const std::string& domain,
                                Operator fn);
 
+        /// \brief      Return the set of names of supported operators.
+        ///
+        /// \param[in]  version  The requested version of ONNX operators set.
+        /// \param[in]  domain   The requested domain the operators where registered for.
+        ///
+        /// \return     The set containing names of supported operators.
+        ///
+        std::set<std::string> get_supported_operators(std::int64_t version,
+                                                      const std::string& domain);
+
         /// \brief Convert an ONNX model to nGraph functions
         /// The function translated serialized ONNX model to nGraph functions. The serialized
         /// ONNX model is read from input stream.