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 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.

Un-skip the proof-of-concept model matrix and drive it through the real
pipeline, keeping both the contiguous and the channels-last case sets, and fold
the focused pipeline coverage into the same suite. The channels-last cases pin
that the pipeline leaves an already-channels-last graph alone.

Known gap, tracked separately: the matrix currently ends at 156 permutes against
138 before the pass. The surviving copies are two thirds boundary, so the method
I/O contract is the fix rather than more peephole fusion.

Local Corstone-300 run of the full Cortex-M suite, the transforms suites, and
lintrunner.

Authored with Codex.

[rascani]

Stack from ghstack (oldest at bottom):

• Cortex-M: add an opt-in NHWC method I/O contract #22029
• 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: absorb boundary layout copies into the method contract #22034
• Channels last: optimize structural layout regions #21962
• Channels last: teach the data-movement passes both permute dialects #22003

[pytorch-bot]

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❌ 1 New Failure, 5 Unrelated Failures

As of commit ccfdfc6 with merge base 8b93850 ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
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👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  .ci/scripts/tests/test_filter_cuda_matrix.py::TestPublishedSets::test_published_python_versions
• pull / unittest / windows / windows-job (gh) (trunk failure)
  .ci/scripts/tests/test_filter_cuda_matrix.py::TestPublishedSets::test_published_python_versions
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  .ci/scripts/tests/test_filter_cuda_matrix.py::TestPublishedSets::test_published_python_versions
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  .ci/scripts/tests/test_filter_cuda_matrix.py::TestPublishedSets::test_published_python_versions
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  .ci/scripts/tests/test_filter_cuda_matrix.py::TestPublishedSets::test_published_python_versions

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