Add external Vision Transformer sample

[lyonsno]

Adds an external sample that runs DeiT-Tiny Vision Transformer inference in WebGPU compute shaders and visualizes attention maps as interactive heatmap overlays.

This follows the external sample path suggested in #569, and is related to #350.

External sample: https://lyonsno.github.io/webgpu-vit-attention/
Repository: https://github.com/lyonsno/webgpu-vit-attention

I'm happy to make any changes that may be needed. Please let me know whether this sample is appropriate for inclusion.

[kainino0x]

Thanks!