Add option to save VAD artifacts
# What does this PR do ?

* Adds utility option to save input and output tensors for VAD model inference useful for testing and comparison in isolation from pre/post processing
* Adds support to disable median filtering for VAD post processing, (`None`, `False`, `''`)

**Collection**: ASR

# Changelog 
- Add VAD options to save artifacts and disable median filtering

# Usage
examples/asr/conf/vad/vad_inference_postprocessing.yaml

```yaml
vad:
    parameters:
      smoothing: None
save_artifacts: False
```

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- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
