Public preset `quant_cfg` changed from dict to list in 0.44 (breaking, undocumented?)

[sayakpaul]

Summary

Between nvidia-modelopt 0.43 and 0.44, the quant_cfg field in the public quantization preset configs (e.g. FP8_DEFAULT_CFG, INT8_DEFAULT_CFG, NVFP4_DEFAULT_CFG) changed from a dict to a list. The internal modelopt.torch.quantization.config._default_disabled_quantizer_cfg changed the same way.

Downstream code that composes a quant_cfg by dict-merging these presets/fragments (a natural pattern) now breaks with TypeError: 'list' object is not a mapping. I couldn't find this called out in the changelog / migration notes — is it intentional, and is there a recommended migration path?

Environment

• nvidia-modelopt: 0.45.0 (also reproduces on 0.44.0; works on 0.43.0)
• torch: 2.14.0.dev20260704+cu130 (the objects involved are plain Python, so this is torch-independent)
• python: 3.12

Minimal reproducer (modelopt only)

import modelopt.torch.quantization as mtq

# `quant_cfg` in the public preset configs is a dict on <=0.43 and a list on >=0.44:
print(type(mtq.FP8_DEFAULT_CFG["quant_cfg"]))             # 0.43: <class 'dict'> | 0.44+: <class 'list'>
print(type(mtq.config._default_disabled_quantizer_cfg))   # 0.43: <class 'dict'> | 0.44+: <class 'list'>

# Code that composes a quant_cfg by dict-merging these now raises:
base = {"*weight_quantizer": {"fake_quant": False}}
merged = {**base, **mtq.config._default_disabled_quantizer_cfg}
# TypeError: 'list' object is not a mapping

Note

mtq.quantize(model, quant_cfg=<dict>, ...) still accepts the old dict-style quant_cfg on 0.45, so the documented quantize path itself is intact — the break is specifically for code that reads/merges the preset quant_cfg objects (or the _default_disabled_quantizer_cfg fragment) as mappings.

Questions

1. Is the dict → list change to preset quant_cfg intentional and stable going forward?
2. Is there a supported/public way to obtain the "default disabled quantizers" fragment (i.e. a replacement for config._default_disabled_quantizer_cfg)?
3. Any migration guidance for merging user overrides into a preset quant_cfg under the new list format?

Context: this surfaced in 🤗 diffusers, whose ModelOpt integration composes quant_cfg from these fragments.

[jenchen13]

Hi @sayakpaul , the change of quant_cfg from from dict to list was in the 0.44 CHANGELOT.rst

Backward Breaking Changes

The quant_cfg field in quantization configs is now an ordered list of QuantizerCfgEntry dicts instead of a flat dictionary. 
...

We also have documentation on the new list-based quant_cfg in the docs. The change was motivated to improve granularity and ordering in quant configs -- for example now the last config entry wins: "If two entries both match *weight_quantizer and both carry a cfg, the second entry does not inherit the first entry’s settings — it replaces them entirely."

[shengliangxu]

Thanks @sayakpaul for reporting. Thanks @jenchen13 for following up.

The change was more than an improvement, it was to fix the semantic. The old dict format is ambiguous when multiple config wildcard items config the same parameter because dict keys are unordered. The list defines a clear ordering and make the semantic deterministic.

However we still accept dict configs for backward compatibility as much as possible by normalizing to list. But old style dict manipulation code will break because of the change of type.

[sayakpaul]

Would it be possible to open a PR to Diffusers to fix this?

[shengliangxu]

Thanks @sayakpaul , absolutely, I'll submit a PR to fix it soon.