Merge pull request #24476 from fengyuentau:attention_layer

dnn: add attention layer #24476

Resolves #24609

Merge with: https://github.com/opencv/opencv_extra/pull/1128.

Attention operator spec from onnxruntime: https://github.com/microsoft/onnxruntime/blob/v1.16.1/docs/ContribOperators.md#com.microsoft.Attention.

TODO:
- [x] benchmark (before this PR vs. with this PR vs. ORT).
- [x] Layer fusion: Take care Slice with end=INT64_MAX.
- [x] Layer fusion: match more potential attention (VIT) patterns.
    - [x] Single-head attention is supported.
- [x] Test AttentionSubgraph fusion.
- [x] Add acc tests for VIT_B_32 and VitTrack
- [x] Add perf tests for VIT_B_32 and VitTrack

## Benchmarks

Platform: Macbook Air M1.

### Attention Subgraph

Input scale: [1, 197, 768].

|                        | mean (ms) | median (ms) | min (ms) |
| ---------------------- | --------- | ----------- | -------- |
| w/ Attention (this PR) | 3.75      | 3.68        | 3.22     |
| w/o Attention          | 9.06      | 9.01        | 8.24     |
| ORT (python)           | 4.32      | 2.63        | 2.50     |

### ViTs

All data in millisecond (ms).

| ViTs     | With Attention | Without Attention | ORT    |
| -------- | -------------- | ----------------- | ------ |
| vit_b_16 | 302.77         | 365.35            | 109.70 |
| vit_b_32 | 89.92          | 116.22            | 30.36  |
| vit_l_16 | 1593.32        | 1730.74           | 419.92 |
| vit_l_32 | 468.11         | 577.41            | 134.12 |
| VitTrack | 3.80           | 3.87              | 2.25   |

### Pull Request Readiness Checklist

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/init.cpp

@@ -162,6 +162,7 @@ void initializeLayerFactory()
     CV_DNN_REGISTER_LAYER_CLASS(LayerNormalization, LayerNormLayer);
     CV_DNN_REGISTER_LAYER_CLASS(Expand,         ExpandLayer);
     CV_DNN_REGISTER_LAYER_CLASS(InstanceNormalization, InstanceNormLayer);
+    CV_DNN_REGISTER_LAYER_CLASS(Attention,      AttentionLayer);
 
     CV_DNN_REGISTER_LAYER_CLASS(Crop,           CropLayer);
     CV_DNN_REGISTER_LAYER_CLASS(Eltwise,        EltwiseLayer);