Add special tokens to existing (trained) SentencePiece models
Signed-off-by: Abhinav Khattar <aklife97@gmail.com>

# What does this PR do ?

Script to add special tokens to existing SentencePiece model. This can help in moving away from the legacy way of handling special tokens.

**Collection**: None

# Usage
`python edit_spt_model.py --input_file <input_model_dir> --output_file <output_model_dir> --tokens <space separated special tokens> `

# Before your PR is "Ready for review"
**Pre checks**:
- [ ] 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

