[FIX] Remove tf backend for generic regularizer in unbalanced

[rflamary]

Types of changes

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.83%. Comparing base (44fd091) to head (191d5f9).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #824   +/-   ##
=======================================
  Coverage   96.82%   96.83%           
=======================================
  Files         124      124           
  Lines       24536    24538    +2     
=======================================
+ Hits        23757    23761    +4     
+ Misses        779      777    -2     

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