Prompt Learning Inference Improvements
Signed-off-by: Virginia Adams <vadams@nvidia.com>

Separates the gpt prompt learning eval script from the gpt model eval script to simplify inference script files and match t5 prompt learning inference workflow. 

**Collection**: NLP

# Changelog 
- Adds megatron_gpt_prompt_learning_eval.py script
- Adds prompt learning inference config
- Updated megatron_gpt_eval.py script
- Updates gpt inference config
- Updated gpt prompt learning dataset class
- Updated gpt prompt learning model class generate and predict step methods


# Usage
```
python megatron_gpt_prompt_learning_eval.py \
	virtual_prompt_model_file=models/squad_p_tuning_126m_tokens100_epochs100_layers4_1e-4.nemo \
        gpt_model_file=models/gpt_126m.nemo \
        inference.greedy=True \
        inference.add_BOS=False \
        trainer.devices=1 \
        tensor_model_parallel_size=1 \
        data_paths=["data/squad_short_test.jsonl"]

python megatron_gpt_prompt_learning_eval.py \
	virtual_prompt_model_file=models/squad_prompt_tuning_5b_tokens100_steps35000_2e-4.nemo \
        gpt_model_file=models/gpt_5b.nemo \
        inference.greedy=True \
        inference.add_BOS=False \
        trainer.devices=2 \
        tensor_model_parallel_size=2 \
        data_paths=["data/squad_short_test.jsonl"]

```

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- [x] Did you add or update any necessary documentation?
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- [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [x] Bugfix
- [ ] Documentation

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# Additional Information
* Related to # (issue)
