Update finetune label models
# What does this PR do ?

Upgraded speaker recognition fine-tune script

**Collection**: [ASR - Speaker Recognition]

# Changelog 
- Removed set_up_finetune_model definition in label_models
- Updated speaker_reco_finetune.py script, to resemble PTL training method
- Now, fine-tune can be directly used from config. Added titanet-finetune.yaml config
- Updated notebook tutorials for the fine-tuning section

# Usage
* You can potentially add a usage example below

```bash
python speaker_reco_finetune.py 
```

# 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?
- [x] 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
- [x] Documentation
