Channels last: preserve metadata across op replacement

[rascani]

Preserve complete operator metadata when replacing ATen operators with channels-last dialect counterparts. ExportPass retracing recomputes layout-dependent shape fields while source attribution, quantization annotations, and custom backend metadata survive.

Document that callers must reject or remap semantic metadata tied to dimensions changed by the replacement specification, such as per-channel quantization axes.

pytest -q backends/transforms/test/test_replace_ops_with_channels_last_variants.py (23 passed).

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]

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