[ONNX] Add interpret_as_scalar helper (#2825)

* Add interpret_as_scalar helper

* Add interpret_as_scalar to OneHot

* clang-format

* Remove interpret_as_scalar from OneHot

We currently only support OneHot with constant depth

* Review comments

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

@@ -28,6 +28,7 @@
 #include "ngraph/op/subtract.hpp"
 #include "ngraph/op/util/broadcasting.hpp"
 #include "onehot.hpp"
+#include "utils/reshape.hpp"
 
 namespace ngraph
 {

src/ngraph/frontend/onnx_import/utils/reshape.cpp

@@ -201,6 +201,24 @@ namespace ngraph
                 return split(node, length_parts, axis_to_split);
             }
 
+            std::shared_ptr<ngraph::Node>
+                interpret_as_scalar(const std::shared_ptr<ngraph::Node>& node)
+            {
+                Shape node_shape = node->get_shape();
+
+                // If node is already a scalar, return original
+                if (node_shape.empty())
+                {
+                    return node;
+                }
+
+                NGRAPH_CHECK((shape_size(node_shape) == 1),
+                             "Scalar value can't be derived from a node with ",
+                             node_shape);
+
+                return ngraph::op::util::reshape(node, Shape{});
+            }
+
         } // namespace  reshape
     }     // namespace onnx_import
 } // namespace ngraph

src/ngraph/frontend/onnx_import/utils/reshape.hpp

@@ -115,6 +115,19 @@ namespace ngraph
                              std::size_t split_parts,
                              int axis = 0);
 
+            /// \brief      Handle a node which represents a scalar value.
+            ///
+            /// \note       Some ONNX nodes, which should provide scalar values are given as
+            ///             tensors of shape {1}. This function will provide a reshape of
+            ///             such a node with Shape{1} into a scalar with Shape{}.
+            ///
+            /// \param[in]  node   Node to reshape.
+            ///
+            /// \return     Original node or a node representing a reshape of the original.
+            ///
+            std::shared_ptr<ngraph::Node>
+                interpret_as_scalar(const std::shared_ptr<ngraph::Node>& node);
+
         } // namespace  reshape
     }     // namespace onnx_import
 } // namespace ngraph