revert back to saving and loading a TFM with torch rather than pickle

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Fixes #2687, fixes #2681

Summary

• fixes errors when loading a torch model that was trained on another device, than the one that is loaded on
• revert back to saving / loading the TFM wrapper with torch rather than pickle

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.09%. Comparing base (4de0868) to head (925a2bc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2692      +/-   ##
==========================================
- Coverage   94.17%   94.09%   -0.08%     
==========================================
  Files         141      141              
  Lines       15579    15582       +3     
==========================================
- Hits        14671    14662       -9     
- Misses        908      920      +12     

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