Add non-mutating quantized weight export API

[mxinO]

Summary

• add a public, non-mutating API for exporting one quantized weight from opaque ModelOpt state
• keep packing and Hugging Face quantization config generation owned by ModelOpt
• support state synchronization, replication, and compatibility checks needed by distributed streaming exporters
• preserve grouped-expert quantizer state during restore

Initial scope

Dynamic-block NVFP4 W4A4 and W4A16. Unsupported state topologies fail explicitly.

Validation

• 12 focused export/unit tests passed
• ruff check and format passed
• aggregate implementation review completed with no material findings

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[github-actions]

PR Preview Action v1.8.1
🚀 View preview at https://NVIDIA.github.io/Model-Optimizer/pr-preview/pr-2193/
Built to branch gh-pages at 2026-08-14 12:52 UTC. Preview will be ready when the GitHub Pages deployment is complete.

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.82%. Comparing base (a57fb44) to head (e58473a).

Files with missing lines	Patch %	Lines
modelopt/torch/export/quantized_weight.py	94.92%	7 Missing ⚠️
modelopt/torch/quantization/plugins/custom.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2193      +/-   ##
==========================================
+ Coverage   78.78%   78.82%   +0.04%     
==========================================
  Files         522      523       +1     
  Lines       60476    60622     +146     
==========================================
+ Hits        47644    47786     +142     
- Misses      12832    12836       +4     

Flag	Coverage Δ
unit	55.46% <94.59%> (+0.13%)	⬆️

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