[slimtensor] Add non-owning storage mode for wrapping external memory

[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

Add non-owning constructor to MaybeOwningStorage that allows wrapping
external memory without taking ownership. This is needed for from_blob()
which creates tensors from user-provided data pointers.

Changes:

• Add MaybeOwningStorage(device, data, nbytes) non-owning constructor
• Add is_owning() method to query ownership mode
• Non-owning storage uses no-op deleter so external memory is not freed

Differential Revision: D89943727

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 7676b63 with merge base 4f8dbde ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Test Metal Backend / export-model-metal-artifact (openai, whisper-large-v3-turbo, non-quantized) / macos-job (gh) (similar failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Test Metal Backend / test-executorch-metal-build / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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