[bugfix] fix partition context unpatch (#7566)
## Fix asymmetric patching/unpatching in
InsertPostInitMethodToModuleSubClasses
### Problem Description
The `InsertPostInitMethodToModuleSubClasses` context manager patches
`__init__` methods of model classes during entry and unpatches them
during exit.
However, asymmetric condition checks between patching and unpatching can
introduce subtle inheritance bugs.
### Root Cause Analysis
The issue occurs with classes that have multiple inheritance where:
1. **Child class A** does not override `__init__`
2. **Parent class B** does not inherit from `nn.Module`
3. **Parent class C** inherits from `nn.Module`
**Current asymmetric logic:**
```python
# Patching (entry): Only patch classes with explicit __init__
def _enable_class(cls):
if '__init__' in cls.__dict__: # ✅ Strict check
cls._old_init = cls.__init__
cls.__init__ = partition_after(cls.__init__)
# Unpatching (exit): Restore any class with _old_init
def _disable_class(cls):
if hasattr(cls, '_old_init'): # ❌ Permissive check
cls.__init__ = cls._old_init
```
**Execution flow:**
1. **During entry**: Child A is skipped (no explicit `__init__`), Parent
C is patched
2. **During exit**: Child A inherits `_old_init` from Parent C and gets
incorrectly "restored"
**Result**: Child A's `__init__` points to Parent C's original
`__init__`, bypassing Parent B and breaking the inheritance chain.
### Reproduction Case
This pattern is common in Hugging Face models:
```python
class Qwen3ForSequenceClassification(GenericForSequenceClassification, Qwen3PreTrainedModel):
pass # No explicit __init__
# GenericForSequenceClassification - not a nn.Module subclass
# Qwen3PreTrainedModel - inherits from nn.Module
```
### Solution
Apply symmetric condition checking in both patch and unpatch operations:
```python
def _disable_class(cls):
# Match the patching condition: only restore classes we explicitly patched
if '__init__' in cls.__dict__ and hasattr(cls, '_old_init'):
cls.__init__ = cls._old_init
delattr(cls, '_old_init') # Optional cleanup
```
This ensures that only classes that were explicitly patched during entry
get restored during exit.
### Testing
The fix has been validated against the Qwen3ForSequenceClassification
reproduction case and resolves the inheritance chain corruption.
### Related Issues
- External issue: https://github.com/modelscope/ms-swift/pull/5820
Co-authored-by: Masahiro Tanaka <81312776+tohtana@users.noreply.github.com>
