Cortex-M: share layout-aware CMSIS-NN kernels

[rascani]

Refactor convolution, depthwise convolution, transpose convolution, average pooling, and max pooling behind shared internal implementations that accept a private activation-layout mode.

Existing registered operators continue selecting the legacy NCHW-logical contract over channels-last storage. This commit changes no public schema or AOT lowering and provides the common implementation used by the following fixed-NHWC wrappers.

59 default-mode dialect tests, MobileNetV3 lowering, and unmodified main-built legacy PTEs for all five refactored operators.

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 643cd15 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 3f0032ccab8e31c6019978c13877b78facefea9a0ca07364bf878185869f3339 /exec failed with exit code 3
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t bfc48c4c7ba63c07a94d7f0751173644481a99fe3c99cfc3533c3796c33a7757 /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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