Cube: variable reusability + refactor in cube macros

[louisfd]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 46 lines in your changes missing coverage. Please review.

Project coverage is 86.07%. Comparing base (5de1517) to head (cd8802e).
Report is 4 commits behind head on main.

Files	Patch %	Lines
crates/burn-cube/tests/frontend/redeclare.rs	79.74%	32 Missing ⚠️
crates/burn-cube-macros/src/tracker.rs	95.28%	5 Missing ⚠️
...ates/burn-cube-macros/src/codegen_function/base.rs	89.65%	3 Missing ⚠️
crates/burn-jit/src/kernel/matmul/simple.rs	0.00%	3 Missing ⚠️
crates/burn-cube-macros/src/analyzer.rs	92.30%	2 Missing ⚠️
.../burn-cube-macros/src/codegen_function/function.rs	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1885      +/-   ##
==========================================
- Coverage   86.14%   86.07%   -0.07%     
==========================================
  Files         778      781       +3     
  Lines       90814    91616     +802     
==========================================
+ Hits        78232    78861     +629     
- Misses      12582    12755     +173     

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