Merge pull request #25613 from CNOCycle:tflite/ops

Support Global_Pool_2D ops in .tflite model #25613

### Pull Request Readiness Checklist

**Merge with extra**: https://github.com/opencv/opencv_extra/pull/1180

This PR adds support for `GlobalAveragePooling2D` and `GlobalMaxPool2D` on the TFlite backend. When the k`eep_dims` option is enabled, the output is a 2D tensor, necessitating the inclusion of an additional flatten layer. Additionally, the names of these layers have been updated to match the output tensor names generated by `generate.py` from the opencv_extra repository.

- [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
- [ ] 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/tflite/tflite_importer.cpp

@@ -71,6 +71,7 @@ private:
     void parseSoftmax(const Operator& op, const std::string& opcode, LayerParams& layerParams);
     void parseCast(const Operator& op, const std::string& opcode, LayerParams& layerParams);
     void parseTranspose(const Operator& op, const std::string& opcode, LayerParams& layerParams);
+    void parseGlobalPooling(const Operator& op, const std::string& opcode, LayerParams& layerParams);
 
     void parseFusedActivation(const Operator& op, ActivationFunctionType activ);
     void parseActivation(const Operator& op, const std::string& opcode, LayerParams& layerParams, bool isFused);
@@ -78,6 +79,8 @@ private:
     int addPermuteLayer(const std::vector<int>& order, const std::string& permName, const std::pair<int, int>& inpId, int dtype);
     int addReshapeLayer(const std::vector<int>& shape, int axis, int num_axes,
                         const std::string& name, const std::pair<int, int>& inpId, int dtype);
+    int addFlattenLayer(int axis, int end_axis, const std::string& name, const std::pair<int, int>& inpId, int dtype);
+
     inline bool isInt8(const Operator& op);
     inline void getQuantParams(const Operator& op, float& inpScale, int& inpZero, float& outScale, int& outZero);
 };
@@ -286,6 +289,7 @@ TFLiteImporter::DispatchMap TFLiteImporter::buildDispatchMap()
     dispatch["CAST"] = &TFLiteImporter::parseCast;
     dispatch["TFLite_Detection_PostProcess"] = &TFLiteImporter::parseDetectionPostProcess;
     dispatch["TRANSPOSE"] = &TFLiteImporter::parseTranspose;
+    dispatch["MEAN"] = dispatch["REDUCE_MAX"] = &TFLiteImporter::parseGlobalPooling;
     return dispatch;
 }
 
@@ -764,6 +768,37 @@ void TFLiteImporter::parseTranspose(const Operator& op, const std::string& opcod
     addLayer(layerParams, op);
 }
 
+void TFLiteImporter::parseGlobalPooling(const Operator& op, const std::string& opcode, LayerParams& layerParams)
+{
+    layerParams.type = "Pooling";
+    if(opcode == "MEAN") {
+        layerParams.set("pool", "ave");
+    }
+    else if (opcode == "REDUCE_MAX") {
+        layerParams.set("pool", "max");
+    }
+    else {
+        CV_Error(Error::StsNotImplemented, "Unsupported pooling " + opcode);
+    }
+    layerParams.set("global_pooling", true);
+    auto options = op.builtin_options_as_ReducerOptions();
+    bool keep_dims = options->keep_dims();
+
+    if (!keep_dims) {
+        const auto name = layerParams.name;
+        layerParams.name += "/global_pooling";
+        addLayer(layerParams, op);
+
+        int out = op.outputs()->Get(0);
+        auto outId = layerIds[out];
+        int flattenId = addFlattenLayer(1, -1, name, outId, isInt8(op) ? CV_8S : CV_32F);
+        layerIds[out] = std::make_pair(flattenId, 0);
+    }
+    else {
+        addLayer(layerParams, op);
+    }
+}
+
 int TFLiteImporter::addPermuteLayer(const std::vector<int>& order, const std::string& permName,
                                     const std::pair<int, int>& inpId, int dtype)
 {
@@ -786,6 +821,16 @@ int TFLiteImporter::addReshapeLayer(const std::vector<int>& shape, int axis, int
     return id;
 }
 
+int TFLiteImporter::addFlattenLayer(int axis, int end_axis, const std::string& name, const std::pair<int, int>& inpId, int dtype)
+{
+    LayerParams lp;
+    lp.set("axis", axis);
+    lp.set("end_axis", end_axis);
+    int id = dstNet.addLayer(name, "Flatten", dtype, lp);
+    dstNet.connect(inpId.first, inpId.second, id, 0);
+    return id;
+}
+
 void TFLiteImporter::parseDeconvolution(const Operator& op, const std::string& opcode, LayerParams& layerParams) {
     layerParams.type = "Deconvolution";
 

modules/dnn/test/test_tflite_importer.cpp

@@ -260,6 +260,14 @@ TEST_P(Test_TFLite, permute) {
     testLayer("permutation_4d_0231");
 }
 
+TEST_P(Test_TFLite, global_average_pooling_2d) {
+    testLayer("global_average_pooling_2d");
+}
+
+TEST_P(Test_TFLite, global_max_pooling_2d) {
+    testLayer("global_max_pooling_2d");
+}
+
 INSTANTIATE_TEST_CASE_P(/**/, Test_TFLite, dnnBackendsAndTargets());
 
 }}  // namespace