Diffusion export bug fixed for model_index.json (#901)

## What does this PR do?

**Type of change:** Bug fix <!-- Use one of the following: Bug fix, new
feature, new example, new tests, documentation. -->

**Overview:** 

Updated diffusers export to preserve the original model_index.json
instead of always rebuilding a minimal one.
The export now uses a simple fallback order: copy original
model_index.json from source path if available, otherwise call
`pipe.save_config(export_dir)`, and only then generate a minimal
model_index.json as last resort.
Non-diffusers export behavior is unchanged.

## Usage
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```python
# Add a code snippet demonstrating how to use this
```

## Testing
<!-- Mention how have you tested your change if applicable. -->

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- **Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/Model-Optimizer/blob/main/CONTRIBUTING.md)**
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- **Is this change backward compatible?**: Yes/No <!--- If No, explain
why. -->
- **Did you write any new necessary tests?**: Yes/No
- **Did you add or update any necessary documentation?**: Yes/No
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[Changelog](https://github.com/NVIDIA/Model-Optimizer/blob/main/CHANGELOG.rst)?**:
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## Additional Information
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved Diffusers pipeline export with enhanced model configuration
handling. The export process now better preserves original pipeline
configurations and uses fallback strategies to ensure complete
configuration files are generated.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jingyu Xin <jingyux@nvidia.com>

modelopt/torch/export/unified_export_hf.py

@@ -937,21 +937,38 @@ def _export_diffusers_checkpoint(
 
             print(f"  Saved to: {component_export_dir}")
 
-    # Step 5: For pipelines, also save the model_index.json
+    # Step 5: For pipelines, also save model_index.json
     if is_diffusers_pipe:
         model_index_path = export_dir / "model_index.json"
-        if hasattr(pipe, "config") and pipe.config is not None:
-            # Save a simplified model_index.json that points to the exported components
+        is_partial_export = components is not None
+
+        # For full export, preserve original model_index.json when possible.
+        # For partial export, skip this to avoid listing non-exported components.
+        if not is_partial_export:
+            source_path = getattr(pipe, "name_or_path", None) or getattr(
+                getattr(pipe, "config", None), "_name_or_path", None
+            )
+            if source_path:
+                candidate_model_index = Path(source_path) / "model_index.json"
+                if candidate_model_index.exists():
+                    with open(candidate_model_index) as file:
+                        model_index = json.load(file)
+                    with open(model_index_path, "w") as file:
+                        json.dump(model_index, file, indent=4)
+
+        # Full-export fallback to Diffusers-native config serialization.
+        # Partial export skips this for the same reason as above.
+        if not is_partial_export and not model_index_path.exists() and hasattr(pipe, "save_config"):
+            pipe.save_config(export_dir)
+
+        # Last resort: synthesize a minimal model_index.json from exported components.
+        if not model_index_path.exists() and hasattr(pipe, "config") and pipe.config is not None:
             model_index = {
                 "_class_name": type(pipe).__name__,
                 "_diffusers_version": diffusers.__version__,
             }
-            # Add component class names for all components
-            # Use the base library name (e.g., "diffusers", "transformers") instead of
-            # the full module path, as expected by diffusers pipeline loading
             for name, comp in all_components.items():
                 module = type(comp).__module__
-                # Extract base library name (first part of module path)
                 library = module.split(".")[0]
                 model_index[name] = [library, type(comp).__name__]