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

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [slimtensor] integration into backend #16565
• [slimtensor] Add from_etensor factory function for ETensor to SlimTensor conversion #16551
• rename slimtensor fiies to consistent with executorch files #16469
• [slimtensor] Add utility functions to common_shims_slim #16457
• [slimtensor] Add storage and device property getters to common_shims_slim #16455
• [slimtensor] Add common_shims_slim with basic property getters #16454
• [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] 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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❌ 1 New Failure, 1 Cancelled Job, 1 Unrelated Failure

As of commit 3adc3b0 with merge base 35eb01a ():

NEW FAILURE - The following job has failed:

• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t a66d667c1dc2840ef321a88b28247572c0f13e0604f25524efe77c288d33dde1 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / test-samsung-models-linux / linux-job (gh)
  ##[error]The operation was canceled.

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)

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