Cortex-M: add explicit-layout runtime primitives

[rascani]

Summary

Define the experimental NHWC operator contracts and reference implementations needed by explicit-layout lowering. Add and register the contiguous-padding runtime kernel, and allow the existing quantized add and multiply channel-broadcast path to operate on both legacy channels-last tensors and contiguous NHWC tensors.

Keep runtime shape and layout validation aligned with the AOT eligibility checks, and make pooling configuration and scratch sizing interpret dimensions through an explicit activation-layout contract.

Why these changes

Explicit layout stores NHWC tensors as ordinary contiguous tensors, so the legacy padding operator cannot infer the intended logical axes from dim order. A distinct pad_contiguous contract makes that difference visible and is registered here alongside its C++ implementation.

Quantized add and multiply already support a per-channel CMSIS-NN loop when the broadcast operand is contiguous in physical memory. The implementation previously identified the loop length through the legacy logical channel axis. This PR instead derives it from the broadcast operand itself, allowing the same path to serve contiguous NHWC tensors while preserving legacy behavior. General and scalar broadcasting remain out of scope.

The remaining NHWC schemas establish Python fake and reference contracts for the following kernel PRs. Their runtime registrations land with their C++ implementations so each kernel PR is independently executable.

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[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 365c550 with merge base 1cfcdf8 ():

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

• pull / test-moshi-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/libs/libssh/libssh-gcrypt-4_0.9.6-2ubuntu0.22.04.7_amd64.deb 404 Not Found [IP: 185.125.190.82 80]

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (trunk failure)

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