Update frozen model config with amp_o2=False:
Signed-off-by: Virginia Adams <vadams@nvidia.com>

Amp_O2 for prompt learning is not supported in the upcoming JoC release. So, megatron_amp_o2 needs to be overwritten and set to False in the GPT model configs that were trained with O2 in order for those models to load for prompt learning.
This will cause a time overhead as the GPT .nemo file will need to be unpacked once to get the config and overwrite it, then unpacked again when the model is restored.

**Collection**: BigNLP

# Changelog 
- Updated PromptLearningGPTModel class to over write megatron amp O2 config.

# 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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