Support Global_Pool_2D ops in .tflite model

[CNOCycle]

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 keep_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

[CNOCycle]

@dkurt

Thank you for your comment. However, I'm not sure which case you're concerned about. Could you please provide your expected model architecture for me to verify the correctness.

[dkurt]

@CNOCycle, never mind. I misunderstood the fact that Flatten goes after Pooling, not before.

I added minor changes to the code to make it look more unified with other layers. LGTM after tests pass.