MLP Prompt Learning  Encoder
# What does this PR do ?

Adds MLP option for prompt learning encoder

**Collection**: NLP

# Changelog 
- Updates Prompt Encoder class
- Updates GPT Prompt Learning Model Class
- Updates Base Prompt Learning Model Class

# Usage
These can optionally be added to the prompt learning config

```
p_tuning: # P-tuning specific params
    encoder_type: "mlp" 
    encoder_hidden: 512
    num_layers: 2
    dropout: 0.0
```

# 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)
- [x] 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

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)
