[slimtensor] Add factory functions for creating empty CPU tensors

[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 adds factory functions to create SlimTensor instances with allocated storage for better usage.

Key components:

1. factory/Empty.h - Factory functions for creating empty tensors:

   • empty_strided(sizes, strides, dtype, device) - Creates tensor with explicit sizes/strides
   • empty(sizes, dtype, device) - Creates contiguous tensor (auto-computes row-major strides)
   • empty(initializer_list, dtype, device) - Convenience overload for inline size specification
   • empty_like(tensor) - Creates empty tensor with same metadata as another tensor

2. core/Storage.h - Added new_storage() helper function:

   • Computes required storage bytes from sizes, strides, and dtype
   • Allocates and returns a new owning Storage instance

Differential Revision: D89820292

[pytorch-bot]

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❌ 1 New Failure, 2 Cancelled Jobs, 1 Unrelated Failure

As of commit 3cecf47 with merge base 9a30bd3 ():

NEW FAILURE - The following job has failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 8e763321892e9c99d65d16fe1c456240c74a349b9cf5eeaa6f629bec875c16a1 /exec failed with exit code 139

CANCELLED JOBS - The following jobs were cancelled. Please retry:

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