Registry interface for custom quantization functional backend

[realAsma]

What does this PR do?

Type of change: ?

Overview:

Add registry interface for custom quantization functional backend

Usage

see tests/unit/torch/quantization/test_custom_backend.py for usage example.

Testing

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• 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?: Yes/No
• Did you add or update any necessary documentation?: Yes
• Did you update Changelog?: Yes

Additional Information

[codecov]

Codecov Report

❌ Patch coverage is 70.58824% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.69%. Comparing base (3fd8b80) to head (cdbb918).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
modelopt/torch/quantization/tensor_quant.py	0.00%	14 Missing ⚠️
.../torch/quantization/nn/modules/tensor_quantizer.py	91.42%	3 Missing ⚠️
modelopt/torch/utils/logging.py	33.33%	2 Missing ⚠️
modelopt/torch/utils/tensor.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #683      +/-   ##
==========================================
+ Coverage   74.65%   74.69%   +0.03%     
==========================================
  Files         192      192              
  Lines       18909    18946      +37     
==========================================
+ Hits        14117    14152      +35     
- Misses       4792     4794       +2     

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[meenchen]

LGTM overall, one question for model restore, do users need to register custom backends before restoring models?

[realAsma]

#683 (review)

Yes, the backend info is stored in the modelopt state. After restore if this backend is not found, an error will be raised.