Added answer only loss for prompt learning
Signed-off-by: Virginia Adams <vadams@nvidia.com>

Adds ability to calculate loss on answer portion of the prompt only for p-tuning/prompt-tuning.


**Collection**: BigNLP

Updated 
- GPTPromptLearningDataset class
- MegatronGPTPromptLearningModel class
- megatron_gpt_prompt_learning_config

# Usage
Same as before, but task templates in the config now have ``answer_only_loss`` and ``answer_field`` added to them.
They are added to the config like:

```python
- taskname: 'rte'
  prompt_template: '<|VIRTUAL_PROMPT_0|>{text}{answer}'
  total_virtual_tokens: 100 
  virtual_token_splits: [100]
  truncate_field: null
  answer_only_loss: True
  answer_field: 'answer'
```

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**PR Type**:
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- [ ] Bugfix
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# Additional Information
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