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.

Read the whole of remove_permutes_around_elementwise_ops.py rather than the
diff: the boundary machinery is easier to follow as a unit than as a patch
against the pre-existing subgraph walk.

Local Corstone-300 run of the full Cortex-M suite (670 passed, 4 skipped, 45
xfailed, 4 xpassed), the transforms suites, and lintrunner.

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: make quantized add and multiply layout-agnostic #21966
• Cortex-M: add a contiguous-layout pad operator #22028
• 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

[pytorch-bot]

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