[Core] return_config=True now extracts just config, not full tarfile
# What does this PR do ?

Make return_config=True now only extract out the yaml files from the tarfile, preventing opening and file extractions of all files (should significantly speedup return_config=True for Billion parameter models).

**Collection**: [Core, ASR, NLP, TTS]

# Changelog 
- Add flag to extract just config from core internals

# Usage

```python
model_config = ModelClass.restore_from(..., return_config=True)
OR
model_config = ModelClass.from_pretrained(..., return_config=True)
```

# 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)
- [x] 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

