Fix Samsung backend PassManager import broken by #20025

[shoumikhin]

#20025 ("Arm backend: Migrate pass manager to exported program") changed exir/passes/__init__.py to re-export ExportedProgramPassManager instead of PassManager. The Samsung backend still imports PassManager from executorch.exir.passes, so every Samsung model export now fails at import:

ImportError: cannot import name 'PassManager' from 'executorch.exir.passes'

This has made test-samsung-quantmodels-linux red on every main commit since 2026-06-11 14:52 UTC and is blocking viable/strict promotion (stale ~31h).

The legacy PassManager class still exists in executorch.exir.pass_manager. The Samsung backend runs passes on a GraphModule (enn_preprocess_passes(edge_program.graph_module)), which is exactly what the legacy PassManager is for, not the new ExportedProgramPassManager. This repoints the import to its canonical module and restores the previous behavior.

cc @usamahz

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Pull request overview

This PR fixes a Samsung backend import regression caused by #20025 changing executorch.exir.passes to no longer re-export the legacy PassManager. It restores Samsung export functionality by importing PassManager from its canonical module (executorch.exir.pass_manager), preserving the existing GraphModule-based pass pipeline behavior.

Changes:

• Update Samsung backend to import PassManager from executorch.exir.pass_manager instead of executorch.exir.passes.

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