Add reduce mean ONNX op support

[laggui]

Related Issues/PRs

Progress towards #1544

Changes

Added reduce mean ONNX op import support

Testing

Added unit tests for codegen and expected outputs

[codecov]

Codecov Report

Attention: Patch coverage is 94.82759% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 86.34%. Comparing base (e303e31) to head (6805081).

Files	Patch %	Lines
crates/burn-import/src/onnx/op_configuration.rs	84.84%	5 Missing ⚠️
crates/burn-import/src/burn/node/unary.rs	98.18%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1637      +/-   ##
==========================================
+ Coverage   86.30%   86.34%   +0.03%     
==========================================
  Files         691      691              
  Lines       79390    79503     +113     
==========================================
+ Hits        68520    68646     +126     
+ Misses      10870    10857      -13     

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[antimora]

LGTM. Thanks for adding new ops.

[nathanielsimard]

LGTM