Fix Samsung backend PassManager import broken by #20025 (#20228)

#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

Author: shoumikhin

backends/samsung/enn_preprocess.py

@@ -39,7 +39,7 @@
     PreprocessResult,
 )
 
-from executorch.exir.passes import PassManager
+from executorch.exir.pass_manager import PassManager
 
 from torch.export.exported_program import ExportedProgram