Channels last: add the common layout pipeline

[rascani]

Add ToContiguousChannelsLastPass as the shared fixed-point pipeline for replacing selected operators with channels-last variants and composing existing data-movement transforms.

Backends can restrict anchors, add layout-equivariant operations, choose explicit padding retargeting, and provide a node-level propagation barrier. Reporting covers converted anchors and structural-copy counts and bytes; strict mode rejects skipped anchors, internal copies, and unknown surviving copy sizes.

Replace the skipped proof-of-concept model matrix with pinned per-case and aggregate permute and view counts, plus focused coverage for barriers, dynamic shapes, padding, byte accounting, and idempotence.

Targeted common pipeline and permute suites passed 88 tests.

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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❌ 3 New Failures

As of commit 2d088ca with merge base 1b2838b ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-buck-build-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 00431d3134895914e7424d26759267a28502efc2d51a60673b9be5105bf4f13b /exec failed with exit code 3
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t d7c6ae737db0482de978e0cd300a49da2229db97f965c7e0b94636884af6beb3 /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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