CUDA: fix im2col_3d to respect non-contiguous inputs (views) (#15956)

CUDA: fix im2col_3d to respect non-contiguous inputs (views) (#15956)

* fix im2col_3d to respect non-contiguous inputs (views)

The CUDA 3D im2col kernel computed source addresses assuming compact layout (products of dims), ignoring nb[] strides.

This patch switches im2col_3d source indexing to use true strides derived from src1->nb[] (in elements), mirroring the approach used in the 2D CUDA im2col path. Destination indexing is unchanged.

* use ggml_element_size() for src strides

Co-authored-by: Johannes Gäßler <redacted>
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Co-authored-by: Johannes Gäßler <redacted>