Add finetuning scripts
# What does this PR do ?

Add a finetuning script along with config file for finetuning existing nemo models. 

**Collection**: ASR

# Changelog 
- Added speech_to_text_finetune.py script
- Added speech_to_text_finetune.yaml config 
- Jenkins run
- Doc strings

TODO:
* Update tutorial

# Usage
Eg: without changing tokenizer
```bash
 python speech_to_text_finetune.py \
 init_from_pretrained_model=stt_en_fastconformer_hybrid_large_pc \
 model.tokenizer.update_tokenizer=false
```
or
finetune with a new tokenizer (architecture of base model doesn't change)
```bash
 python speech_to_text_finetune.py  \
 init_from_pretrained_model=stt_en_fastconformer_hybrid_large_pc \
 model.tokenizer.update_tokenizer=true \
 model.tokenizer.dir=<path_to_tokenizer_dir> \
 model.tokenizer.type=<bpe/char>
```


# 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 