[slimtensor] Introduce CUDA guard to aoti/slim/cuda

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [slimtensor] Add aoti_torch_item_bool for SlimTensor #16453
• [slimtensor] Add aoti_torch_assign_tensors_out for SlimTensor #16452
• [slimtensor] Add aoti_torch_copy_ for SlimTensor #16451
• [slimtensor] Add aoti_torch__reinterpret_tensor for SlimTensor #16450
• [slimtensor] Add aoti_torch_new_tensor_handle for SlimTensor #16449
• [slimtensor] Add aoti_torch_delete_tensor_object for SlimTensor #16448
• [slimtensor] Add SlimTensor-based aoti_torch_empty_strided #16447
• [slimtensor] Add SlimTensor-based aoti_torch_create_tensor_from_blob_v2 #16446
• -> [slimtensor] Introduce CUDA guard to aoti/slim/cuda #16724
• [slimtensor] Add item() methods with CUDA and CPU support for scalar value extraction #16445
• [slimtensor] Add permute() and reshape() view operations #16444
• [slimtensor] Add as_strided() view operation for tensor reinterpretation #16443
• [slimtensor] Add from_blob() factory function for wrapping external memory #16442
• [slimtensor] Add non-owning storage mode for wrapping external memory #16441
• [slimtensor] Enable CUDA tensor copy #16440
• [slimtensor] Add CUDA slimtensor creation with basic functionality #16439
• [slimtensor] Add CUDA Storage with DeviceTraits and memory allocation #16438
• [slimtensor] Add CUDA DeviceType and extend Device class #16437
• [slimtensor] Add all required dtype support (Int8/16/32/64, Bool, BFloat16) #16399
• [slimtensor] Add factory functions for creating empty CPU tensors #16398

Copy CUDAGuard and CUDAStreamGuard from cuda/runtime/ to aoti/slim/cuda/
to support slimtensor requirement while get rid of potential circular dependency:

• cuda_backend/main_functionalities -> aoti/slimtensor -> cuda_backend/cuda_guard

This change:

• copy guard.h, guard.cpp and test files from backend/cuda_backend to backend/aoti/slim/cuda/

Differential Revision: D91056808

[pytorch-bot]

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❌ 1 New Failure

As of commit b655367 with merge base 4f8dbde ():

NEW FAILURE - The following job has failed:

• pull / test-coreml-bc-macos (macos-m2-stable) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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