defer tirex import in tests

[dennisbader]

Fixes failing unit tests by deferring torch import

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.25%. Comparing base (16fda02) to head (4d89601).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3091      +/-   ##
==========================================
- Coverage   96.30%   96.25%   -0.05%     
==========================================
  Files         160      161       +1     
  Lines       17280    17346      +66     
==========================================
+ Hits        16642    16697      +55     
- Misses        638      649      +11     

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