XNNPACK graph runtime: core tensor and quantization types

[GregoryComer]

Foundational data types for the new XNNPACK graph runtime: Tensor/Storage,
DType, Span, a variant helper, and quantization parameters. No behavior
change — these sources compile into xnnpack_backend but are not yet reachable.

Authored with Claude.

[GregoryComer]

Stack from ghstack (oldest at bottom):

• XNNPACK graph runtime: backend integration behind use_graph_runtime #20256
• XNNPACK graph runtime: flatbuffer graph deserialization #20255
• XNNPACK graph runtime: executor engine #20254
• XNNPACK graph runtime: memory planning #20253
• XNNPACK graph runtime: execution planning #20252
• XNNPACK graph runtime: partitioner #20251
• XNNPACK graph runtime: arena allocator and shape environment #20250
• XNNPACK graph runtime: in-tree operator interface #20249
• XNNPACK graph runtime: graph IR and builder #20248
• -> XNNPACK graph runtime: core tensor and quantization types #20247

[pytorch-bot]

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❌ 11 New Failures, 1 Unclassified Failure

As of commit 7cc1262 with merge base 4c9c444 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  /pytorch/executorch/backends/xnnpack/runtime/core/quant_params.cpp:195:21: error: use of undeclared identifier 'num_scales'
• Cadence Build & Test / hifi-build / hifi4 (gh)
  Process completed with exit code 1.
• Cadence Build & Test / vision-build / vision (gh)
  Process completed with exit code 1.
• pull / android / build-android (gh)
  /home/runner/work/executorch/executorch/backends/xnnpack/runtime/core/tensor.cpp:142:14: error: use of undeclared identifier 'num_elements'
• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 011d9aad16c66d3fd1d1e7a815c7ee085ea2474bbe80c5c7ca019988f81d41e9 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t d162e1d816ec84e596871de5bbda2f7261e31cb0a4d2350d1ed3d447f6671a53 /exec failed with exit code 8
• pull / unittest / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 8
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1abde86626e61119c17a5f9b2aea0bb7598586d0475a323972e075a4f59d87fb /exec failed with exit code 8
• pull / unittest-editable / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 8
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh)
  RuntimeError: Command docker exec -t f6b75d2196a45d44ac8c3de6cf42f9973080bacee8cd3239c22bb8b49cc8806e /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t ab3862fe3d8a7fd39beea91b76af3155696c17a53fef0478fe58ee0f14eedd44 /exec failed with exit code 1

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• pull / test-qnn-direct-build-linux / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command docker exec -t 52ff3560a760ba1d799104d24030c1fb1c0fbe19de8afda3b1254863183fb2e7 /exec failed with exit code 127

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