[slimtensor] Add item() methods with CUDA and CPU support for scalar value extraction

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• 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
• [slimtensor] Add SlimTensor class with basic properties and CPU copy operation #16385
• [slimtensor] Add SizesAndStrides and dimension utility functions for slimtensor usage #16384
• [slimtensor] Storage and SharedPtr for CPU owning mode #16383
• [slimtensor] Introduce Device and ScalarType headers for SlimTensor minimal support #16382

Add two item() methods to SlimTensor for extracting scalar values from single-element tensors:

1. item<T>() - Templated version requiring explicit type specification
2. item() - Non-templated version that auto-detects dtype and returns double

Both methods:

• Support all 7 slim dtypes (Float, BFloat16, Long, Int, Short, Char, Bool)
• Handle CUDA tensors by automatically copying data to CPU
• Validate that tensor has exactly 1 element

Differential Revision: D90034340

[pytorch-bot]

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❌ 6 New Failures, 1 Unrelated Failure

As of commit 79fb587 with merge base fd6fa87 ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t bc030d1aa6f9dee5aeee8ce033bdf70a46b37b93a8648b987c8604396679fb22 /exec failed with exit code 139
• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 4c561d4440851840863df23b4e8099fa0c6dad8509f1593f43ab7e4df0eb5248 /exec failed with exit code 1
• pull / test-samsung-quantmodels-linux / linux-job (gh)
  RuntimeError: Command docker exec -t f6d00ea8f1e20c746b84df063d94a3a1756acc3c1c67ac3c271376dc2848b197 /exec failed with exit code 1
• Test CUDA Builds / export-model-cuda-artifact (openai, whisper-small, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 149b0a719533ab00093d7f54c1d640c347c70989276129b38f1a7c7bdec4f0d2 /exec failed with exit code 1
• Test CUDA Builds / export-model-cuda-artifact (openai, whisper-small, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 3399f622ea745bfd7f0ce13a05adc05e7a830aab429af22d42f053ec341e9ba4 /exec failed with exit code 1
• Test CUDA Builds / test-models-cuda (sdpa) / linux-job (gh)
  RuntimeError: No CUDA GPUs are available

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

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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