upper cap pytorch lightning for ckpt symlink fix

[dennisbader]

see #2133

Summary

• fixes currently failing unit tests
• sets an upper cap on pytorch lightning <=2.1.2 to fix the checkpoint symlink issue.
• pytorch lightning already reverted these changes here. Will relax the cap once they release the new version

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ab3d77a) 93.92% compared to head (4567521) 93.91%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2136      +/-   ##
==========================================
- Coverage   93.92%   93.91%   -0.01%     
==========================================
  Files         135      135              
  Lines       13335    13321      -14     
==========================================
- Hits        12525    12511      -14     
  Misses        810      810              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.