[ExecuTorch][WebGPU] Graph-owned scratch buffers for fused-op intermediates

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] SDPA test suite: replay + dynamic input_pos + in-graph KV cache #20087
• [ExecuTorch][WebGPU] Add fused SDPA (sdpa_with_kv_cache) with dynamic input_pos #20086
• [ExecuTorch][WebGPU] SymInt live-scalar mechanism + et_vk.select_as_symint #20085
• [ExecuTorch][WebGPU] Add update_cache tests (native numeric + export) #20084
• [ExecuTorch][WebGPU] Add update_cache op (llama.update_cache) #20083
• [ExecuTorch][WebGPU] Add per-pass dispatch ordering + scratch buffer tests #20080
• [ExecuTorch][WebGPU] Switch native backend from wgpu-native to Dawn (Tint) + SwiftShader #20079
• -> [ExecuTorch][WebGPU] Graph-owned scratch buffers for fused-op intermediates #20073
• [ExecuTorch][WebGPU] Per-pass compute dispatch ordering for fused multi-dispatch ops #20072

Add WebGPUGraph::create_scratch_buffer for fused-op intermediates (SDPA's attn_weights/attn_weights_softmax) that are not model tensors and live only between dispatches. Graph-owned, released in the destructor. Vulkan models these as graph tensors; we use raw buffers (buffer-only backend). Consumed by the fused SDPA op above.

Differential Revision: D107543259

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20073

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure

As of commit ddc439f with merge base f976e63 ():

NEW FAILURE - The following job has failed:

• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t 177e19bb74c33eb30b082aac4cc86b483439f161ad1d77be67ce920280836389 /exec failed with exit code 1

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.

[digantdesai]

Review automatically exported from Phabricator review in Meta.