Fix for prompt table restore error
Signed-off-by: Virginia Adams <vadams@nvidia.com>

Every time a new low val loss is reached, the virtual tokens from the prompt encoder are moved to the prompt table in order to save a .nemo file for that checkpoint. This PR removes the premature virtual prompt tokens from the prompt table so that restoring from a .ckpt doesn't error. The error was:

```
Traceback (most recent call last):
  File "scripts/convert_prompt_learning_ckpt_to_nemo.py", line 85, in main
    model: MegatronGPTPromptLearningModel = MegatronGPTPromptLearningModel.load_from_checkpoint(
  File "/home/vadams/NeMo/nemo/collections/nlp/models/nlp_model.py", line 355, in load_from_checkpoint
    model = ptl_load_state(cls, checkpoint, strict=strict, cfg=cfg, **kwargs)
  File "/home/vadams/miniconda3/envs/dev/lib/python3.8/site-packages/pytorch_lightning/core/saving.py", line 259, in _load_state
    keys = obj.load_state_dict(checkpoint["state_dict"], strict=strict)
  File "/home/vadams/NeMo/nemo/collections/nlp/models/language_modeling/megatron_gpt_prompt_learning_model.py", line 382, in load_state_dict
    self.prompt_table.load_state_dict(state_dict_, strict)
  File "/home/vadams/miniconda3/envs/dev/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1667, in load_state_dict
    raise RuntimeError('Error(s) in loading state_dict for {}:\n\t{}'.format(
RuntimeError: Error(s) in loading state_dict for PromptTable:
	Unexpected key(s) in state_dict: "prompt_table.squad.indices", "prompt_table.squad.prompt_embeddings.weight"
```

**Collection**: NLP

# Changelog 
- Updated GPT and T5 Prompt learning

# Usage
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```python
# Add a code snippet demonstrating how to use this 
```

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# Additional Information
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