Refactor and clean up hf_ptq.py

[shengliangxu]

What does this PR do?

Refactor and clean up hf_ptq.py

This script has several separate logic and the code of them are entangled, making it really hard to add new features

Refactor them so that we separate these logics:

1. sparsity, all logic go to sparsity_main. TODO: we may actually move this logic out to a separate script

2. quantize, all logic go to quantize_main.

   2.1 plain quantization with a single quantization format

   2.2 auto quantization

In the quantization pipeline, separate the pipeline to:

1. model loading
2. calibrate dataset loading
3. pre-quantize processing
4. actual quantize
5. post-quantize processing
6. quantized model export

Testing

tested the plain quantization:

python examples/llm_ptq/hf_ptq.py \
    --pyt_ckpt_path=Qwen/Qwen3-8B \
    --export_path=qwen3-8B_fp8 \
    --qformat=fp8 \
    --kv_cache_qformat=fp8 \
    --calib_size=16 \
    --batch_size=0 \
    --trust_remote_code \
    --export_fmt=hf

tested auto quantize:

python examples/llm_ptq/hf_ptq.py \
    --qformat=nvfp4,fp8 \
    --auto_quantize_score_size 128 \
    --auto_quantize_bits 5.0 \
    --auto_quantize_checkpoint Qwen3-8B-auto-quantize-checkpoint \
    --pyt_ckpt_path=Qwen/Qwen3-8B \
    --export_path=qwen3-8B_auto_quantize \
    --kv_cache_qformat=fp8 \
    --calib_size=16 \
    --batch_size=0 \
    --trust_remote_code \
    --export_fmt=hf

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.45%. Comparing base (f265f8d) to head (e15d632).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #665   +/-   ##
=======================================
  Coverage   74.45%   74.45%           
=======================================
  Files         183      183           
  Lines       18412    18412           
=======================================
  Hits        13709    13709           
  Misses       4703     4703           

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