[OMNIML-2244] Implement the ONNX quantization exporter for INT4

[ajrasane]

What does this PR do?

Type of change:
New Feature

Overview:

• Created an abstract parent class for ONNXQuantExporter
• Created child classes for individual precisions
• Implemented the INT4QuantExporter
• Removed quantize_weights_to_int4
• Added a method to quantize weights of the ONNX model to low precision

Testing

python torch_quant_to_onnx.py --quantize_mode=int4_awq \
	--onnx_save_path=<onnx_path> \

Before your PR is "Ready for review"

• Make sure you read and follow Contributor guidelines and your commits are signed.
• Is this change backward compatible?: Yes
• Did you write any new necessary tests?: No
• Did you add or update any necessary documentation?: Yes
• Did you update Changelog?: No

[codecov]

Codecov Report

❌ Patch coverage is 23.52941% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.32%. Comparing base (a703e22) to head (83028fa).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
modelopt/torch/_deploy/utils/torch_onnx.py	18.75%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #575      +/-   ##
==========================================
- Coverage   74.45%   74.32%   -0.13%     
==========================================
  Files         182      182              
  Lines       18250    18164      -86     
==========================================
- Hits        13588    13501      -87     
- Misses       4662     4663       +1     

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[gcunhase]

If this PR is just for INT4, and NVFP4 and MXFP8 are WIP, can you please update the title accordingly? Thanks!