PTQ example for NeMo 2.0
# What does this PR do ?

Example workflow for quantizing NeMo 2.0 checkpoint.

**Collection**: llm

# Known Issues 

* Observed accuracy regression on MMLU when exporting a non-quantized LLAMA3-7B: `0.654 -> 0.638`
* Half-precision with PP>1 does not work

# Usage
* You can potentially add a usage example below

```python
from nemo.collections.llm import quantization

quantization_config = quantization.QuantizationConfig()
export_config = quantization.ExportConfig('/tmp/trt_llama_engine')
quantizer = quantization.Quantizer(quantization_config, export_config)
model = quantization.load_with_modelopt_layer_spec('/path/to/nemo/checkpoint')
model = quantizer.quantize(model)
quantizer.export(model)
```

# GitHub Actions CI

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# Before your PR is "Ready for review"
**Pre checks**:
- [ ] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [ ] Bugfix
- [ ] Documentation

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## Who can review?

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[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
