Extract shared device test utilities to reduce redundancy (#20061)

[Gasoonjia]

Summary:

Extract DeviceAwarePartitioner, CpuOnlyPartitioner, and MockCudaAllocator into shared test utility modules to eliminate duplicated definitions across test files.

Python: Create executorch/exir/backend/test/device_util.py with DeviceAwarePartitioner (configurable target_device, default "cuda:0"), CpuOnlyPartitioner, and AddOperatorSupport. Update 3 consumer test files.

C++: Create executorch/runtime/core/test/mock_cuda_allocator.h with a canonical MockCudaAllocator (malloc/free/memcpy-backed, with call tracking). Update 4 consumer test files.

Reviewed By: abhinaykukkadapu

Differential Revision: D99925172

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20061

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❌ 2 Unclassified Failures

As of commit 7ff6067 with merge base 12684ef ():

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Test CUDA Builds / export-model-cuda-artifact (SocialLocalMobile, gemma-4-31B-it-HQQ-INT4, quantized-int4-tile-packed) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• Test CUDA Builds / export-model-cuda-artifact (SocialLocalMobile, Qwen3.5-35B-A3B-HQQ-INT4, quantized-int4-tile-packed) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)

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