[slimtensor] Enable CUDA tensor copy

[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

This diff enables CUDA tensor copy operations in SlimTensor.

Key changes:

core/SlimTensor.h - Extended for CUDA support:

• Updated copy_() to handle cross-device copies:
  • CPU→CUDA (cudaMemcpyHostToDevice)
  • CUDA→CPU (cudaMemcpyDeviceToHost)
  • CUDA→CUDA (cudaMemcpyDeviceToDevice, same device)
• Cross-device copies require contiguous tensors
• CPU-to-CPU copies continue to support non-contiguous (strided) tensors

Differential Revision: D89941235

[pytorch-bot]

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❌ 4 New Failures

As of commit 9801804 with merge base 4f8dbde ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t e27b437025ad5381e5f403830081cfe1d68e6983e681602427ebb3bed0b649ed /exec failed with exit code 139
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model
• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t dcc3414ef815f1eb05948e3302dbff50a5b280c076f8f173cfc9c835702f4dc9 /exec failed with exit code 1
• Test Metal Backend / test-model-metal-e2e (openai, whisper-small, non-quantized) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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