Continuous prompt refactor
# What does this PR do ?

Refactor to combine p-tuning and prompt tuning  code as well as integrate it with the new evaluation code/api. 
Prompt tuning and p-tuning are now jointly called 'prompt learning' within NeMo. 

**Collection**: BigNLP

# Changelog 
- Added MegatronGPTPromptLearningModel class
- Added GPTPromptLearningDataset class, merging p-tuning and prompt-tuning dataset classes
- Removed p-tuning dataset and model classes
- Removed prompt tuning code from the MegatronGPTModel, GPTModel, and TransformerLanguageModel classes
- Edited inference/generation code to accommodate p-tuning/prompt-tuning 

# Usage

```python
python megatron_gpt_prompt_learning.py --config-name=megatron_gpt_prompt_learning_config.yaml
python megatron_gpt_eval.py --config-name=megatron_gpt_inference.yaml
```

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# Additional Information
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