Fix categorical nans

[loreloc]

• Removed unnecessary transpositions in the Categorical index-based implementation
• Replaced Categorical one-hot implementation with the index-based as it is generally faster and more memory efficient anyway

Closes #311 .

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 68.18%. Comparing base (4bb4617) to head (669958c).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
cirkit/backend/torch/layers/input.py	42.85%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #317   +/-   ##
=======================================
  Coverage   68.18%   68.18%           
=======================================
  Files          51       51           
  Lines        5366     5366           
  Branches      615      615           
=======================================
  Hits         3659     3659           
  Misses       1491     1491           
  Partials      216      216           

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