[BUG] *Logger fails with StackingEnsemble #458

[martins0n]

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`closes #460`

[codecov-commenter]

Codecov Report

Merging #460 (b09a09c) into master (75fd188) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
+ Coverage   87.97%   87.99%   +0.01%     
==========================================
  Files         115      115              
  Lines        5440     5448       +8     
==========================================
+ Hits         4786     4794       +8     
  Misses        654      654              

Impacted Files	Coverage Δ
etna/ensembles/stacking_ensemble.py	98.31% <100.00%> (+0.01%)	⬆️
etna/loggers/base.py	95.77% <100.00%> (+0.46%)	⬆️

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[julia-shenshina]

thanks! very important fix ⭐