ggml : fix conv2d_dw SVE path (#1380)

ggml : fix conv2d_dw SVE path (#1380)

* Fix test-conv2d-dw failure on ARM SVE by using runtime vector length

The ggml_compute_forward_conv_2d_dw_cwhn function was using a hardcoded GGML_F32_EPR (8) for SIMD vectorization, but on ARM SVE the actual vector length varies by hardware. This caused incorrect computation when processing CWHN layout tensors on ARM machines.

Fix by using svcntw() to get the runtime SVE vector length instead of the compile-time constant.

Co-authored-by: ggerganov <redacted>
* ci : reduce sam score threshold

* ci : update bbox checks for sam test

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Co-authored-by: copilot-swe-agent[bot] <redacted>
Co-authored-by: ggerganov <redacted>