[slim tensor migration 2/n] update slimtensor namespace

[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 updates namespaces of slimtensor files to executorch standard. More specific:

1. namespace standalone::slim → namespace executorch::backends::aoti::slim
2. namespace standalone::c10 → namespace executorch::backends::aoti::slim::c10
3. Added torch::executor::SlimTensor backward-compatible alias

Differential Revision: D89442123

[pytorch-bot]

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

As of commit 723e60b 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 7d3c12b87567c2cd1683d58514c953597253a1ec85ed4866b83fa67fd31e0b5e /exec failed with exit code 1
• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  RuntimeError: Command docker exec -t dbfd8983f8ddbc27def0603df14b1598b6cfd7ae996698e11cb5e0b7ee81695b /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 9e5362315ccef06a26e4a40e2057c134aacf1030a26dc806414cb75f5cc321b7 /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 job failed but was likely due to flakiness present on trunk:

• pull / unittest-buck / linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  Error response from daemon: Get "https://308535385114.dkr.ecr.us-east-1.amazonaws.com/v2/": context deadline exceeded

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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