Inherit all PerSegment models from PerSegmentModel

[alex-hse-repository]

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• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Related Issue

Closing issues

closes #508

[codecov-commenter]

Codecov Report

Merging #543 (1f49044) into master (ab902ba) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #543      +/-   ##
==========================================
- Coverage   87.28%   87.26%   -0.02%     
==========================================
  Files         118      118              
  Lines        5797     5788       -9     
==========================================
- Hits         5060     5051       -9     
  Misses        737      737              

Impacted Files	Coverage Δ
etna/models/base.py	77.44% <100.00%> (ø)
etna/models/catboost.py	100.00% <100.00%> (ø)
etna/models/holt_winters.py	98.76% <100.00%> (+0.01%)	⬆️
etna/models/seasonal_ma.py	97.05% <100.00%> (+0.08%)	⬆️
etna/models/sklearn.py	96.07% <100.00%> (-0.75%)	⬇️

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