XNNPACK graph runtime: flatbuffer graph deserialization

[GregoryComer]

Adds FlatbufferGraphBuilder, which deserializes the XNNPACK .pte payload into
the in-memory graph IR and reports external input/output ids.

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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❌ 6 New Failures

As of commit d4a23ff with merge base 4c9c444 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 0a127a105565dc04f60b99b251e18bfd182eae9dbfc4c030dcc2c8149085764e /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 389a8be2e4aeba46c952159e2794e2a49312ca291199e54c48cf0aa4f4f4b986 /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 db2d90e9b4ad46dda285e28e4df29fef57742c6194cb45f0b2c8eb2086091674 /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t dd27bb3847f3b3d1300c17784c1c97a92f6964537879d039e32a377a281c4af0 /exec failed with exit code 1

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