[ET Device Support] Define et_copy runtime h2d and d2h copy ops

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [ET Device Support] CUDA-native Qwen 3.5 MoE inference with device tensor pipeline #18788
• [ET Device Support] Extract shared device test utilities to reduce redundancy #18762
• [ET Device Support] Add device tensor helper functions to TensorPtr API #19828
• [ET Device Support] make device support config method-based #19582
• [ET Device Support] Add ExecutorchBackendConfig flags for skipping H2D/D2H copies #18760
• [ET Device Support] PropagateDevicePass inserts H2D/D2H copy ops at delegate boundaries #18730
• -> [ET Device Support] Define et_copy runtime h2d and d2h copy ops #18729
• [ET Device Support] Add device tensor helper functions to TensorPtr API #18761

Implement C++ runtime kernels for device copy ops using DeviceAllocator:

• h2d_copy_out: infers device from out tensor, calls
  DeviceAllocator::copy_host_to_device
• d2h_copy_out: infers device from self tensor, calls
  DeviceAllocator::copy_device_to_host
• Registered via EXECUTORCH_LIBRARY macro

Differential Revision: D99636776

[pytorch-bot]

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• pull / test-static-llama-qnn-linux (stories_110m) / linux-job (gh) (trunk failure)
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• Test CUDA Builds / export-model-cuda-artifact (SocialLocalMobile, Qwen3.5-35B-A3B-HQQ-INT4, quantized-int4-tile-packed) / linux-job (gh) (trunk failure)
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