[slimtensor] Storage and SharedPtr for CPU owning mode

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [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

This diff introduces the foundation for tensor storage management in the SlimTensor migration:

• util/SharedPtr.h - A lightweight, non-thread-safe shared pointer optimized for single-threaded tensor operations.
• core/Storage.h - The MaybeOwningStorage class that manages tensor data memory:
  • DeviceTraits specialization with allocate(), free(), memcpy() using malloc/free
  • Owning mode for CPU device (CUDA and non-owning mode added later)
  • Storage type alias as SharedPtr
  • Move semantics for efficient resource transfer
  • clone() and copy_() methods for data management

Differential Revision: D89747980

[pytorch-bot]

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❌ 1 New Failure, 1 Cancelled Job, 1 Pending, 1 Unrelated Failure

As of commit 5a10dcc with merge base 9a30bd3 ():

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 a2277523a966dec2d9a6348f4af83e01cab1ce4f468e115972754058aaf8c349 /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)
  Timeout waiting for emulator to boot.

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