[ET Device Support] Add ExecutorchBackendConfig flags for skipping H2D/D2H copies

[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

Add skip_h2d_for_method_inputs and skip_d2h_for_method_outputs config
flags to ExecutorchBackendConfig. These control whether PropagateDevicePass
skips inserting H2D/D2H copy ops at method I/O boundaries:

• skip_h2d_for_method_inputs: user provides GPU tensor directly
• skip_d2h_for_method_outputs: output stays on device for cross-method pipelines

Differential Revision: D99636778

[pytorch-bot]

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As of commit 31ef072 with merge base 77df9b7 ():

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• pull / android / build-android (gh)

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• pull / test-static-llama-qnn-linux (stories_110m) / linux-job (gh) (trunk failure)
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[JacobSzwejbka]

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