Replace some powf->powi

[ArthurBrussee]

Changes

Replace some powf -> powi calls. This isn't any faster currently, but could be in the future if powi is specialized.

[laggui]

Think the macos runner isn't picking up the job because other runners are already hanging.. looks like it might have something to do with the new cubecl config.

[codecov]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.34%. Comparing base (5a437b0) to head (31c00ad).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-core/src/optim/rmsprop.rs	66.66%	1 Missing ⚠️
crates/burn-tensor/src/tensor/api/numeric.rs	66.66%	1 Missing ⚠️
crates/burn-tensor/src/tensor/ops/int_tensor.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3152      +/-   ##
==========================================
+ Coverage   81.32%   81.34%   +0.01%     
==========================================
  Files         817      818       +1     
  Lines      117804   117827      +23     
==========================================
+ Hits        95802    95842      +40     
+ Misses      22002    21985      -17     

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