Merge pull request #29624 from abhishek-gola:linear_flex_attention_layers

Linear and Flex attention layers support - #29624

### 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/misc/python/test/test_onnx_conformance.py

@@ -27,8 +27,12 @@ TOLERANCE_OVERRIDES = {
     "test_attention_4d_gqa_with_past_and_present_fp16_expanded": (0.0002, 0.001),
     "test_causal_conv_with_state_fp16": (0.0002, 0.002),
     "test_causal_conv_with_state_silu_fp16": (0.0002, 0.002),
+    "test_flexattention_fp16": (0.0002, 0.001),
+    "test_flexattention_fp16_expanded_ver26": (0.0002, 0.001),
     "test_gelu_tanh_1": (0.00011, 0.00016),
     "test_gelu_tanh_2": (9e-05, 0.0005),
+    "test_linear_attention_fp16": (0.0002, 0.001),
+    "test_linear_attention_fp16_expanded": (0.0002, 0.001),
     "test_nllloss_NCd1d2_reduction_sum_expanded": (2e-05, 0.0001),
     "test_nllloss_NCd1d2d3d4d5_mean_weight_expanded": (2e-05, 0.0001),
     "test_reduce_prod_default_axes_keepdims_random": (0.002, 0.002),