Simplify logic with variadic_add and variadic_mul helpers

[ricardoV94]

Just a small cleanup. The need to know whether we are adding / multiplying 0, 1, or more inputs logic shows up in several places (also in our downstream packages) that I think we're better of just adding a helper

This change revealed a bug in the grad of TrueDiv that was also fixed, and I cleaned up the insane test that revealed this error (the error would still be found with the new test)

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 81.75%. Comparing base (1c2bc8f) to head (86e4283).
Report is 108 commits behind head on main.

Files with missing lines	Patch %	Lines
pytensor/scalar/basic.py	53.84%	6 Missing ⚠️
pytensor/tensor/blas.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #932      +/-   ##
==========================================
+ Coverage   81.73%   81.75%   +0.01%     
==========================================
  Files         183      183              
  Lines       47734    47708      -26     
  Branches    11611    11589      -22     
==========================================
- Hits        39016    39003      -13     
+ Misses       6523     6514       -9     
+ Partials     2195     2191       -4     

Files with missing lines	Coverage Δ
pytensor/ifelse.py	51.70% <100.00%> (ø)
pytensor/tensor/basic.py	91.54% <100.00%> (ø)
pytensor/tensor/math.py	91.13% <100.00%> (-0.16%)	⬇️
pytensor/tensor/rewriting/basic.py	94.11% <100.00%> (-0.04%)	⬇️
pytensor/tensor/rewriting/blas.py	88.16% <100.00%> (-0.05%)	⬇️
pytensor/tensor/rewriting/math.py	89.81% <100.00%> (+0.39%)	⬆️
pytensor/tensor/rewriting/subtensor.py	90.32% <100.00%> (+0.20%)	⬆️
pytensor/tensor/subtensor.py	89.32% <100.00%> (ø)
pytensor/tensor/blas.py	65.54% <50.00%> (+0.17%)	⬆️
pytensor/scalar/basic.py	80.41% <53.84%> (ø)

... and 6 files with indirect coverage changes