Fix circular import in cortex_m passes using relative imports (pytorch#16376)

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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Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests)
for ExecuTorch PR guidelines.

[PLEASE REMOVE] If this PR closes an issue, please add a `Fixes
#<issue-id>` line.

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Requests](https://github.com/pytorch/executorch/blob/main/CONTRIBUTING.md#pull-requests).

### Test plan
[PLEASE REMOVE] How did you test this PR? Please write down any manual
commands you used and note down tests that you have written if
applicable.

Co-authored-by: Github Executorch <github_executorch@arm.com>

Author: psiddh

backends/cortex_m/passes/cortex_m_pass_manager.py

@@ -10,14 +10,6 @@
     FoldAndAnnotateQParamsPass,
     ScalarsToAttributePass,
 )
-from executorch.backends.cortex_m.passes import (
-    ActivationFusionPass,
-    ClampHardswishPass,
-    ConvertToCortexMPass,
-    DecomposeHardswishPass,
-    QuantizedOpFusionPass,
-    ReplaceQuantNodesPass,
-)
 from executorch.backends.transforms.replace_scalar_with_tensor import (
     ReplaceScalarWithTensorArgPass,
 )
@@ -26,6 +18,13 @@
 from executorch.exir.program._program import _transform
 from torch.export import ExportedProgram
 
+from .activation_fusion_pass import ActivationFusionPass
+from .clamp_hardswish_pass import ClampHardswishPass
+from .convert_to_cortex_m_pass import ConvertToCortexMPass
+from .decompose_hardswish_pass import DecomposeHardswishPass
+from .quantized_op_fusion_pass import QuantizedOpFusionPass
+from .replace_quant_nodes_pass import ReplaceQuantNodesPass
+
 
 class CortexMPassManager(PassManager):
 

backends/cortex_m/quantizer/__init__.py

@@ -16,4 +16,3 @@
     SOFTMAX_OUTPUT_FIXED_QSPEC,
     SOFTMAX_PER_TENSOR_CONFIG,
 )
-from .quantizer import CortexMQuantizer, SharedQspecQuantizer  # noqa