Cortex-M: add opt-in explicit-layout lowering

[rascani]

Summary

Add the opt-in explicit-layout lowering pipeline for Cortex-M.

CortexMQuantizer(use_explicit_layout=True) selects the explicit convolution eligibility checks and includes Conv1d annotation before it is widened to Conv2d. CortexMPassManager(use_explicit_layout=True) selects a separate, ordered pass list that forms NHWC spatial regions, removes redundant local layout copies, validates the resulting graph, and lowers to the experimental NHWC Cortex-M operators.

The default quantizer checks and pass list remain unchanged. The mode is selected only at the quantization and lowering boundaries; it is not threaded through individual passes or the target configuration. Unsupported quantized spatial anchors fail closed instead of mixing legacy and explicit operator families.

Test plan

pytest --config-file=backends/arm/test/pytest.ini backends/cortex_m/test/test_explicit_layout_pipeline.py backends/cortex_m/test/misc/test_quantization.py backends/cortex_m/test/misc/test_quantizer_reporter.py

lintrunner -m origin/main

AI-assisted: Codex.

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[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit 1979f91 with merge base c65b23a ():

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

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

• pull / test-samsung-models-linux / linux-job (gh) (trunk failure)
  AttributeError: type object 'Test_Milestone_MobileBertFinetune' has no attribute 'model_cache_dir'

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