Load model in the target export precision by default in PTQ
# What does this PR do ?

Configuring PTQ to use the same precision to load a model as `export.dtype` by default. This is to avoid possible OOM errors when casting weights on the model export step.

This is for use cases with less device memory available, for example, PTQ for Llama3-8b on L4 24GB GPU.

**Collection**: NLP

# Changelog 
- Add specific line by line info of high level changes in this PR.

# Usage

* In order to use FP16 / BF16 weights precision enabling `model.megatron_amp_O2` is necessary
* Disabling `model.dist_ckpt_load_on_device` avoid GPU memory spikes on model loading
* Using a lower `inference.batch_size` (compared to default 64) might also be needed.

Example PTQ command is:
```
python megatron_gpt_ptq.py \
    model.restore_from_path=Llama3-8b \
    +model.dist_ckpt_load_on_device=false \
    export.dtype=bf16 \
    inference.batch_size=16
```

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# Additional Information
* Related to https://github.com/NVIDIA/NeMo/issues/9981