Channels last: optimize structural layout regions

[rascani]

Introduce common structural layout-copy helpers and teach the shared transpose and permute transforms to preserve layout provenance only when every contributing copy is layout-owned.

Add opt-in layout-region boundary propagation with backend barriers, rank and broadcast validation, constant remapping, alias handling, symbolic-shape support, and copy-volume safety. Per-channel QDQ remains a barrier because its axis is dimension-dependent.

Define channels_last constant padding for padding arguments already remapped into NHWC logical order and retain direct portable decomposition.

Targeted channels-last and permute suites passed 124 tests with 12 runtime-serialization cases deselected pending the final runner build.

Authored with Codex.

[rascani]

Stack from ghstack (oldest at bottom):

• Cortex-M: keep NHWC channel broadcasts quantized #21968
• Cortex-M: add opt-in explicit layout lowering #21967
• Cortex-M: add explicit-NHWC channel-broadcast ops #21966
• Cortex-M: add explicit-NHWC runtime operators #21965
• Cortex-M: share layout-aware CMSIS-NN kernels #21964
• Channels last: add the common layout pipeline #21963
• -> Channels last: optimize structural layout regions #21962
• Channels last: preserve metadata across op replacement #21961

[pytorch-bot]

🔗 Helpful Links

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✅ You can merge normally! (1 Unrelated Failure)

As of commit aeacde5 with merge base 1b2838b ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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