Cortex-M: add a contiguous-layout pad operator

[rascani]

Add cortex_m::pad_contiguous beside the existing pad. The two differ only in
whether the kernel has to work out the layout for itself: the existing operator
infers it, while this one requires the dim order to say the tensor is contiguous
and then indexes the padding by logical axis.

Not named after an axis order, because padding does not interpret axes the way
convolution and pooling do -- it pads whatever it is told to. What distinguishes
this entry point is that it never guesses. It accepts ranks one to four for the
same reason, so it can replace the inferring operator outright once serialized
programs no longer need that one, rather than sitting beside it forever.

Leave the existing operator alone. Its layout predicate is tolerant on purpose:
the tolerance short-circuits before the dim order is consulted, which is what
keeps it agreeing with the AOT pass for shapes whose serialized dim order cannot
name the channel axis. Two regression cases cover that, since every pre-existing
channels-last pad test used more than one channel and so missed it.

Local Corstone-300 run of the full Cortex-M suite 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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❌ 2 New Failures, 10 Unrelated Failures, 1 Unclassified Failure

As of commit ea98643 with merge base 8b93850 ():

NEW FAILURES - The following jobs have failed:

• pull / test-mcu-cortex-m-backend / linux-job (gh)
  RuntimeError: Command docker exec -t 632138afab7b0abb9abacafb18b0bf05de09f82bf0248258887983951b51ace5 /exec failed with exit code 1
• trunk / test-models-macos-coreml (w2l) / macos-job (gh)
  torchaudio

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• trunk / test-coreml-delegate / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 128

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

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

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