Add ability to give seperate train test and validation splits
# What does this PR do ?
Gives the user the ability to specify separate train,test and validation datasets as a dictionary in data_prefix for gpt model

**Collection**: nlp/language_modelling

# Changelog 
- Supports dictionary format in data_prefix for megatron gpt config file.
- Uses the existing data loader to load each individual dataset appropriately by changing split string

# Usage
* data_prefix: {train:/path/to/train, test:/path/to/test, validation:/path/to/validation}

# 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
- [ ] Documentation

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


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
