[ExecuTorch][WebGPU] Add et_vk.prepack (constant-tensor packing) for E2E weight loading#20429
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JulianCloudNTH merged 2 commits intoJun 22, 2026
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…E2E weight loading Pull Request resolved: #20265 Adds the WebGPU backend handler for `et_vk.prepack.default`, the node the VulkanPartitioner wraps around every constant feeding a delegated op so the constant is materialized into its dedicated GPU buffer before inference. For the WebGPU backend's buffer-flat/fp32 model, prepack is an identity layout (same dims, dtype, and bytes), so the handler runs no compute shader: it validates that `src` and `out` match (dims, `elem_size`, `nbytes`, non-null buffers; every check throws fail-loud) and records a one-time `src`->`out` buffer-to-buffer copy via the new `WebGPUGraph::add_prepack_copy`. The recorded copies run once in a new `build()` Phase 4 (after the op-dispatch chain is recorded), mirroring the Vulkan delegate's separate `prepack()` init phase (distinct from per-inference `execute()`). Ordering is guaranteed by the WebGPU queue -- the prepack submit precedes the first `execute()` submit on the same queue, so the copied data is visible without an explicit device poll (Dawn has no `wgpuDevicePoll`, and the backend relies on queue ordering plus the output-map wait elsewhere). `src.elem_size` is the `WebGPUTensor` field added by the embedding op lower in this stack, so prepack stacks above it. ghstack-source-id: 395549289 @exported-using-ghexport Differential Revision: [D108428754](https://our.internmc.facebook.com/intern/diff/D108428754/)
🔗 Helpful Links🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20429
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Pull Request resolved: #20292 Test suite for the `et_vk.prepack` constant-materialization op, split into its own diff (op below, tests above) per the per-op test-split convention. The prepack op is how a serialized constant becomes a GPU tensor: the constant arrives as a CPU-side reference (sizes + a pointer into the .pte bytes), and the prepack node is the sole materialization — one CPU->GPU transfer straight into the consumer's buffer. The model `M(x) = x + w` (w a constant) routes `w` through a prepack node, so the delegate must run the materialization for the output to equal `x + w` rather than `x + 0`. ghstack-source-id: 395555139 @exported-using-ghexport Differential Revision: [D108678631](https://our.internmc.facebook.com/intern/diff/D108678631/)
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This PR was created by the merge bot to help merge the original PR into the main branch.
ghstack PR number: #20265 by @JulianCloudNTH
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ghstack PR base: https://github.com/pytorch/executorch/tree/gh/JulianCloudNTH/27/base
ghstack PR head: https://github.com/pytorch/executorch/tree/gh/JulianCloudNTH/27/head
Merge bot PR base: https://github.com/pytorch/executorch/tree/gh/JulianCloudNTH/29/orig
Merge bot PR head: https://github.com/pytorch/executorch/tree/gh/JulianCloudNTH/27/orig
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