XNNPACK graph runtime: graph IR and builder

[GregoryComer]

Graph IR for the new runtime: nodes, operators, tensor specs, typed handles,
and the in-memory GraphBuilder used to construct graphs (and by the unit
tests). Depends only on the core types. Still not wired into the backend.

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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❌ 9 New Failures

As of commit 818766a with merge base 4c9c444 ():

NEW FAILURES - The following jobs have failed:

• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t b52636a028319640edf4cf6433d5951852d3794e1f5363396487d2babfb3528b /exec failed with exit code 1
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 69105288be4ab46bc80ea0ec5c06fa41a9bcaf1bbcb53c22eb52d1cc18a5952d /exec failed with exit code 1
• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh)
  RuntimeError: Command docker exec -t d64d927a38b30c28c12b6c2625f0dc4522863171654efd7b7aea7a82b93b0ced /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 037c1033739671eda285c58e806c67b2825a77dd4f34da09481a9374a5543297 /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 14efe89ed194152ae23b74c47949e200fcceb7ded187f975b60ee1d0651c4bfc /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 4d2764bce4180ed88d1ab8a43cd7be1d62d399ac25b6cc7cac2ade1489ee9e86 /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t 31d24a31b8e5468151e8b0936fc1257c9a78605b11610c791213e411c1bf09f8 /exec failed with exit code 1

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