Fix finetuning complex models correctly unfreezes.

[scart97]

What does this PR do?

This changes the way the nodes are filtered in the FinetuningCallback to avoid the problem with complex models described in the issue.

Fixes #6879

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[codecov]

Codecov Report

Merging #6880 (39efb11) into master (9fbe724) will decrease coverage by 4%.
The diff coverage is 100%.

❗ Current head 39efb11 differs from pull request most recent head f78fa8d. Consider uploading reports for the commit f78fa8d to get more accurate results

@@           Coverage Diff           @@
##           master   #6880    +/-   ##
=======================================
- Coverage      91%     87%    -4%     
=======================================
  Files         193     193            
  Lines       12299   12298     -1     
=======================================
- Hits        11252   10707   -545     
- Misses       1047    1591   +544     

[tchaton]

LGTM ! Thanks for this contribution !