Save Virtual Prompt Weights Only
Removes all GPT/Frozen model configs and weights from Prompt Learning Model's .nemo file. After training has ended, the prompt learning model now only saves prompt table parameters. During training, intermediate checkpoint files may also have prompt encoder parameters. 

**Collection**: BigNLP
- Adds custom state_dict and load_state_dict methods for prompt learning model class
- Updates prompt learning inference example script
- Updates prompt learning documentation

# Usage
The final .nemo checkpoint file contains

1. model_config.yaml
2. model_weights.ckpt

where model_weights.ckpt only has prompt table params

# 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
- [x] 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)
