[slimtensor migration 5/n] reuse et c10 headers

[Gasoonjia]

Stack from ghstack (oldest at bottom):

• [slimtensor migration 6/n] add copyright headers #16330
• -> [slimtensor migration 5/n] reuse et c10 headers #16329
• [slimtensor migration 4/n] use ET ArrayRef and IntArrayRef #16327
• [slimtensor migration 3/n] use et_check instead of standalone check #16314
• [slim tensor migration 2/n] update slimtensor namespace #16313
• [slim tensor migration 1/n] copy and paste slim tensor into ExecuTorch #16304

This stack aims to migrate slim tensor into ExecuTorch stack to make it as internal tensor representation of cudabackend.

This diff makes slimtensor depends on pytorch/pytorch c10 headers living in ExecuTorch space by creating a lean wrapper on it to avoid large amout of migration work.

Differential Revision: D89505483

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/16329

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❌ 2 New Failures, 1 Cancelled Job, 6 Unrelated Failures

As of commit 436ca0a with merge base c61b2ed ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t b388def6295a45e1def3537ea45a68d2c597493d89ebebb92a2b5ff6c7b70b24 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (mistralai, Voxtral-Mini-3B-2507, quantized-int4-tile-packed) / linux-job (gh)
  RuntimeError: Command docker exec -t 018845fd671930d9e7c74fdaf822e811529c2d7a2e2578fd705e5a798a5765be /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.

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-llama-runner-linux (fp32, xnnpack+custom+qe, linux.arm64.2xlarge, executorch-ubuntu-22.04-gc... / linux-job (gh) (matched linux rule in flaky-rules.json)
  The process '/usr/bin/git' failed with exit code 128
• pull / test-static-llama-qnn-linux (stories_260k_bc) / linux-job (gh) (matched linux rule in flaky-rules.json)
  docker: Error response from daemon: Get "https://308535385114.dkr.ecr.us-east-1.amazonaws.com/v2/tool/alpine/manifests/sha256:def822f9851ca422481ec6fee59a9966f12b351c62ccb9aca841526ffaa9f748": dial tcp 52.206.205.216:443: i/o timeout.
• Test CUDA Builds / test-model-cuda-e2e (google, gemma-3-4b-it, non-quantized) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, non-quantized) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, quantized-int4-weight-only) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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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