Enable ONNX export of 5B GPT trained with TE FP8 modules
# What does this PR do ?

Adds export functionality for GPT model trained with FP8 enabled

**Collection**: [NLP]

# Changelog 
- Add export script
- Add export config
- Update export_utils to enable support for this model
- Bugfix: Typecast precision check for 16 and 32

# Usage

```bash
python3 examples/nlp/language_modeling/megatron_gpt_export.py gpt_model_file=5b_fp8_tp1.nemo onnx_model_file=5b_fp8_tp1.onnx
```

# Before your PR is "Ready for review"
**Pre checks**:
- [X] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [X] Did you add or update any necessary documentation?
  
**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [X] Documentation

