Fix QLoRA example test (#553)

## 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:** model.active_adapter() call is deprecated, causing the
example tests to fail. Updated to use the latest call.

## Usage
<!-- You can potentially add a usage example below. -->

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

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

## Before your PR is "*Ready for review*"
<!-- If you haven't finished some of the above items you can still open
`Draft` PR. -->

- **Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/TensorRT-Model-Optimizer/blob/main/CONTRIBUTING.md)**
and your commits are signed.
- **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
- **Did you update
[Changelog](https://github.com/NVIDIA/TensorRT-Model-Optimizer/blob/main/CHANGELOG.rst)?**:
Yes/No <!--- Only for new features, API changes, critical bug fixes or
bw breaking changes. -->

## Additional Information
<!-- E.g. related issue. -->

Signed-off-by: Suguna Velury <[email protected]>

Author: sugunav14

modelopt/torch/quantization/plugins/transformers_trainer.py

@@ -302,7 +302,7 @@ def _load_best_model(self, *args, **kwargs):
                 if "base_layer" in name
             ), "Some base_layer parameters are not frozen"
 
-            adapter_name = self.model.active_adapter()
+            adapter_name = self.model.active_adapters()[0]
             self.model.delete_adapter(adapter_name)
             self.model.load_adapter(self.state.best_model_checkpoint, adapter_name)
         else: