[Speech Data Explorer] An option to explicitly specify the base dir
[Speech Data Explorer] An option to explicitly specify the base dir for manifests with relative paths


**Collection**: Tools

# Changelog 
- Use `args.audio_base_path` instead of `args.manifest`
- If `args.audio_base_path` is not provided (`None`), then default to the location (parent dir) of the manifest

# Usage

```python
python data_explorer.py /path/to/a/modified/manifest.json --audio-base-path /path/to/audio_data
```

# Before your PR is "Ready for review"
**Pre checks**:
- [ 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?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ x] New Feature
- [ ] Bugfix
- [ ] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
