Avoid copying output from GPU to CPU for ASR runner

[larryliu0820]

Summary

This PR adds the ability to skip copying GPU outputs back to CPU for specific methods in the CUDA backend, and enables conditional CUDA compilation for the ASR runner.

Changes

CUDA Backend (backends/cuda/runtime/cuda_backend.cpp)

• Changed the skip_copy_output_to_cpu_for_method backend option from a boolean to a string that accepts a method name
• The option can be set via set_option() after init() is called, allowing runtime configuration
• During execute(), the backend compares the configured method name against the handle's method name to decide whether to skip the GPU→CPU output copy
• Thread-safe access to the skip-copy method name via mutex

Usage example:

BackendOptions<1> options;
options.set_option("skip_copy_output_to_cpu_for_method", "encode");
set_option("CudaBackend", options.view());

AOTI Delegate Handle (backends/aoti/aoti_delegate_handle.h)

• Added method_name field to AOTIDelegateHandle to track which method each delegate handle corresponds to

ASR Runner CMake (extension/asr/runner/CMakeLists.txt)

• Added conditional CUDA support: when EXECUTORCH_BUILD_CUDA is enabled and CUDAToolkit is found, the CUDA_AVAILABLE compile definition is added
• This allows ASR runner code to conditionally compile CUDA-aware code paths

Motivation

When running multi-method models (e.g., prefill + decode for LLMs, or encoder + decoder for ASR), some methods benefit from keeping outputs on GPU to avoid unnecessary memory copies between methods. This change enables fine-grained control over which method(s) skip the GPU→CPU copy.

Test Plan

• Build with CUDA enabled
• Verify skip_copy_output_to_cpu_for_method option works for specified method
• Verify other methods still copy outputs to CPU by default

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16235

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