fix(Phi4Multimodal): Fix incorrect default vision/audio config initialization in Phi4MultimodalConfig
Author: charlieJ107
Pull request: yes

## 🐛 Bug Fix: Phi4MultimodalConfig default sub-config initialization

This PR fixes two issues in `Phi4MultimodalConfig.__init__` related to default initialization of multimodal sub-configs. 

Rations in Phi4MultimodalConfig

# What does this PR do?


### ❌ Problems fixed

1. When `vision_config` is `None`, a default `Phi4MultimodalVisionConfig()` was instantiated but **not assigned**, leaving `self.vision_config` as `None`.
2. The `audio_config` default initialization incorrectly checked `vision_config is None` instead of `audio_config is None`.

### ✅ Changes

```python
if isinstance(vision_config, dict):
    vision_config = Phi4MultimodalVisionConfig(**vision_config)
elif vision_config is None:
    vision_config = Phi4MultimodalVisionConfig()
self.vision_config = vision_config

if isinstance(audio_config, dict):
    audio_config = Phi4MultimodalAudioConfig(**audio_config)
elif audio_config is None:
    audio_config = Phi4MultimodalAudioConfig()
self.audio_config = audio_config
```

### 🎯 Impact

- Ensures `vision_config` and `audio_config` are always valid config objects when omitted
    
- Prevents downstream errors caused by unexpected `None` values
    
- Improves consistency with other multimodal config implementations
    

No behavior change for users explicitly passing configs.


Let me know if you'd like tests added or additional adjustments.
This pull request fixes initialization logic for the vision and audio configuration objects in the `Phi4MultimodalModel` constructor. The changes ensure that default configuration objects are correctly assigned when none are provided.

Configuration initialization fixes:

* Corrected the assignment of default `vision_config` and `audio_config` objects in the `__init__` method of `modular_phi4_multimodal.py`, ensuring that they are properly instantiated when not provided.
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Fixes #43479 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      to it if that's the case.
     Fixing the issue: #43479
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@Cyrilvallez
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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: phi4_multimodal

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