Fix cortex_m test failures from D106339880

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> Fix cortex_m test failures from D106339880 #19783
• [pytorch/executorch][diff_train] Cortex-M backend: Add AoT scratch-buffer planning. (#19636) #19782
• [pytorch/executorch][diff_train] Arm backend: Add bf16 support for aten.index_select and aten.unfold_copy (#19751) #19781
• [pytorch/executorch][diff_train] NXP backend: Enable Sub Tensor with new Neutron flow (#19588) #19780
• [pytorch/executorch][diff_train] Arm backend: Exclude build metadata from license checks #19779
• [pytorch/executorch][diff_train] NXP backend: Enable Add Tensor with new Neutron flow (#19550) #19778

D106339880 introduced two issues that broke modai/test:test_cortex_recipes:

1. convert_to_cortex_m_pass.py imported CortexMPass from executorch.backends.cortex_m.passes, but the Buck cortex_passes library does not include the package's __init__.py, so the package namespace is empty in Buck builds. Switch to a direct module import from cortex_m_pass.

2. scratch_buffer_sizes.py was added but never listed in the Buck cortex_passes srcs. Add it so the import resolves at test time.

3. _CortexMEdgePassManager in modai/recipes/cortex.py only injected exported_program when constructing pass classes. ConvertToCortexMPass now extends CortexMPass, which also requires target_config. Mirror CortexMPassManager.transform() and pass target_config (defaulting to CortexMTargetConfig(cpu=CortexM.M55)) when the pass signature accepts it.

Differential Revision: D106408368

[pytorch-bot]

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❌ 9 Pending, 1 Unrelated Failure, 1 Unclassified Failure

As of commit acf90c6 with merge base fb3f6eb ():

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• Check Labels / Check labels (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: GraphQL query

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

• pull / test-static-llama-qnn-linux (stories_110m) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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