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

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• (to be filled)

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: D91202898

[pytorch-bot]

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❌ 5 New Failures, 1 Unrelated Failure

As of commit fa49848 with merge base 3512861 ():

NEW FAILURES - The following jobs have failed:

• pull / test-coreml-bc-macos (macos-m2-stable) / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 4f2181a751ed7b52224ba47deb3be708d77538452cd9b5c745dc48b8f03743ac /exec failed with exit code 1
• Test Metal Backend / export-model-metal-artifact (mistralai, Voxtral-Mini-3B-2507, non-quantized) / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• Test Metal Backend / export-model-metal-artifact (openai, whisper-small, non-quantized) / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• Test Metal Backend / test-executorch-metal-build / macos-job (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?

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) (matched macos rule in flaky-rules.json)
  File doesn't exist

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