Fix circular import in cortex_m passes using relative imports

[psiddh]

Changed cortex_m_pass_manager.py to use relative imports (from .activation_fusion_pass) instead of absolute imports (from executorch.backends.cortex_m.passes) for intra-package references. This breaks the circular import chain: init.py → cortex_m_pass_manager.py → init.py

Resolves: ImportError: cannot import name 'ActivationFusionPass' from partially initialized module 'executorch.backends.cortex_m.passes'

Summary

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❌ 2 New Failures, 4 Unrelated Failures

As of commit f776870 with merge base 39203cf ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t 107a5384effd2ab0c34cc652a52d47a208a5e47c927cd3a36bbff547af1ee0a6 /exec failed with exit code 1
• trunk / test-mcu-cortex-m-backend / linux-job (gh)
  RuntimeError: Command docker exec -t 42f8a8b29aa9a93ea2ffabe35d50ad85b60f041d8608008478f4c5fecd3dcd26 /exec failed with exit code 1

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-models-linux (add_mul, portable, linux.2xlarge) / 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.
• pull / test-models-linux (emformer_transcribe, xnnpack-quantization-delegation, linux.2xlarge) / 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.
• pull / test-models-linux-basic (vit, xnnpack-quantization-delegation, buck2, linux.2xlarge, executorch-u... / 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.

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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Pull request overview

This PR fixes a circular import error in the cortex_m passes module by converting absolute imports to relative imports in cortex_m_pass_manager.py. The circular dependency chain (__init__.py → cortex_m_pass_manager.py → __init__.py) is broken by having cortex_m_pass_manager.py import directly from individual pass modules rather than through the package's __init__.py.

Key Changes:

• Replaced absolute imports (from executorch.backends.cortex_m.passes import ...) with relative imports (from .activation_fusion_pass import ...) for all six pass classes
• Moved import statements to maintain proper ordering (after external imports, before usage)

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