## 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: modelscope/ms-swift#5820

Co-authored-by: Masahiro Tanaka <[email protected]>