XNNPACK graph runtime: backend integration behind use_graph_runtime

[GregoryComer]

Wires the graph runtime into XnnpackBackend behind the use_graph_runtime
backend option (default off, with per-method runtime-spec override). When
enabled, init() deserializes via FlatbufferGraphBuilder and builds an Executor;
execute()/destroy() gain the graph path alongside the legacy XNNCompiler path.

With prefer_in_tree_kernel disabled, existing all-XNNPACK models run end-to-end
on the new executor.

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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❌ 8 New Failures

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• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 1c3b41115d1bca998f1b87a31a19ff611f0d960d4504fbd3e29898fecef9adff /exec failed with exit code 1
• pull / test-qnn-delegate-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e3f5f2e8f8c7024574b771836368ab767ef189088d11e56d8549984fe8a03b65 /exec failed with exit code 92
• pull / unittest / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch
• 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 ec84d0bef15e986c1712e129473c116e5405efa32ce7c5e7cfabea1066b66b97 /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 cc7dae27bc75eaf56d2f6aa73f36fb584a4e9ecc7ccec6f6e38f4f6c933cbb36 /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t 1b019fb6553d980b261facba8aca69f30a31ef778d27d5308187cc7e350e8414 /exec failed with exit code 1

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