Fix ModelPruning(make_pruning_permanent=True) buffers getting removed when saved during training

[carmocca]

What does this PR do?

During training, every time the model is saved via ModelCheckpoint, if make_pruning_permanent, the buffers would get removed. This means that the buffers would not accumulate and the pruning amounts are incorrect.

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[codecov]

Codecov Report

Merging #6073 (faae25c) into master (352e8f0) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #6073   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         159     159           
  Lines       11362   11370    +8     
======================================
- Hits        10605   10581   -24     
- Misses        757     789   +32     

[tchaton]

LGTM !

[SkafteNicki]

smaller comments

[tchaton]

LGTM !