[Rewriter] Add fuse batchnorm to default rules

[AyoubMDL]

This PR adds fuse_batchnorm rules to default rules.

[Copilot]

Pull Request Overview

This PR adds batch normalization fusion rules to the default rewriter rules, enabling automatic optimization of BatchNorm patterns during model rewriting.

• Refactors _fuse_batchnorm.py to use class variables instead of constructor parameters
• Adds the fuse_batchnorm rules to the default optimization rules in __init__.py

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
onnxscript/rewriter/rules/common/_fuse_batchnorm.py	Refactors classes to use ClassVar for op_type and removes constructors
onnxscript/rewriter/init.py	Imports and includes _fuse_batchnorm rules in default optimization rules

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.34%. Comparing base (647b22a) to head (d3c6f74).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2553      +/-   ##
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- Coverage   70.34%   70.34%   -0.01%     
==========================================
  Files         218      218              
  Lines       26430    26424       -6     
  Branches     2647     2647              
==========================================
- Hits        18593    18587       -6     
  Misses       6934     6934              
  Partials      903      903              

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