Merge pull request #23746 from Abdurrahheem:ash/graph_simplifier

Assertion Fix in Split Layer #23746

### Pull Request Readiness Checklist

This PR fixes issue mentioned in [#23663](https://github.com/opencv/opencv/issues/23663)
Merge with https://github.com/opencv/opencv_extra/pull/1067

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/dnn/src/onnx/onnx_importer.cpp

@@ -1363,7 +1363,14 @@ void ONNXImporter::parseSplit(LayerParams& layerParams, const opencv_onnx::NodeP
     {
         DictValue splits = layerParams.get("split");
         const int numSplits = splits.size();
-        CV_Assert(numSplits > 1);
+
+        if (numSplits == 1)
+        {
+            layerParams.set("num_split", 1);
+        }
+        else
+        {
+            CV_Assert(numSplits >= 1);
 
             std::vector<int> slicePoints(numSplits - 1, splits.get<int>(0));
             for (int i = 1; i < splits.size() - 1; ++i)
@@ -1372,6 +1379,7 @@ void ONNXImporter::parseSplit(LayerParams& layerParams, const opencv_onnx::NodeP
             }
             layerParams.set("slice_point", DictValue::arrayInt(&slicePoints[0], slicePoints.size()));
         }
+    }
     else if (node_proto.input_size() == 2) // opset >= 13, the split will be stored at the second input, instead of the attribute.
     {
         CV_Assert(constBlobs.find(node_proto.input(1)) != constBlobs.end());

modules/dnn/test/test_onnx_importer.cpp

@@ -1146,6 +1146,7 @@ TEST_P(Test_ONNX_layers, Split)
         applyTestTag(CV_TEST_TAG_DNN_SKIP_IE_NN_BUILDER);
     if (backend == DNN_BACKEND_INFERENCE_ENGINE_NGRAPH)
         applyTestTag(CV_TEST_TAG_DNN_SKIP_IE_NGRAPH);
+    testONNXModels("split_0");
     testONNXModels("split_1");
     testONNXModels("split_2");
     testONNXModels("split_3");