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

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch][WebGPU] Add et_vk.prepack (constant-tensor packing) for E2E weight loading #20265
• [ExecuTorch][WebGPU] Add et_vk.apply_rotary_emb (interleaved RoPE) + ValueList multi-output #20264
• [ExecuTorch][WebGPU] Add et_vk.embedding_q4gsw (4-bit groupwise-symmetric quantized embedding) #20263

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.
@exported-using-ghexport

Differential Revision: D108428754

Differential Revision: D108428754

[pytorch-bot]

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❌ 4 New Failures, 15 Pending, 1 Unrelated Failure

As of commit 76c146d with merge base 0378fc4 ():

NEW FAILURES - The following jobs have failed:

• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 2174fbb0f58a1b6ff07ee14b8febc4c937e2b185f67927993512eb8c59dffb6d /exec failed with exit code 1
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 794ebd1f109c0d98dc3b22374ad57b0969372cd08c2d8479f3792c8abbe5cd7a /exec failed with exit code 1
• pull / test-qnn-models-linux (dl3) / linux-job (gh)
  RuntimeError: Command docker exec -t d8ca226ec0fb062e3aea24f68b4bbd7754c415840dbcd5cd965f2447fdfd5207 /exec failed with exit code 92
• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh)
  RuntimeError: Command docker exec -t 71041c11cc9f19d6ad6262f826046bc80046432dd7491ee936ac0f18b35e2348 /exec failed with exit code 92

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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