[ExecuTorch][WebGPU] Add et_vk.prepack (constant-tensor packing) for E2E weight loading#20429

2 commits merged into

gh/JulianCloudNTH/29/origpytorch/executorch:gh/JulianCloudNTH/29/origfrom

gh/JulianCloudNTH/27/origpytorch/executorch:gh/JulianCloudNTH/27/origCopy head branch name to clipboard