Fix GPTSFTDataset
# What does this PR do ?
* Change middle tokenization logic from tokenization/detokenization to separate prompt template 
* Add multiple contexts and truncation fields
* Add truncation augmentation

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage
_D.R suggest we document the changes here, e.g._ 

**Fixes** 
- Remove the old logic prompt_template -> tokenize -> compute total tokens-> truncate -> detokenize -> prompt_template -> tokenize
- Add new logic prompt_template -> separate prompt_template -> tokenize -> compute total tokens -> truncate
- In separate prompt_template, we need to carefully adding spaces to the next token
- In separate prompt_template,  we need to separate from left to right
-
**New features**
Remove query_key and make context_key  have multiple keys by a ,  for example context_key="context,query"
Make truncation_field can have multiple keys, logic the same as context_key
Add multiple truncation if we have multiple truncation_field . Current logic is to equally truncate each field
Add truncation augmentation (random truncate)

```python
# Add a code snippet demonstrating how to use this 
```

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- [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
- [x] Bugfix
- [ ] Documentation

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


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# Additional Information
* Related to # (issue)
