use cpu version of PyTorch for testing

[ArneBinder]

by adding torch again as dev dependency, but with source = "pytorch-cpu" and

[[tool.poetry.source]]
name = "pytorch-cpu"
url = "https://download.pytorch.org/whl/cpu"
priority = "explicit"

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.20%. Comparing base (fdde528) to head (73ede54).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #10   +/-   ##
=======================================
  Coverage   97.20%   97.20%           
=======================================
  Files          24       24           
  Lines        2004     2004           
=======================================
  Hits         1948     1948           
  Misses         56       56           

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