Create AbstractPipeline

[alex-hse-repository]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

Before submitting (must do checklist)

• 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 #560

[codecov-commenter]

Codecov Report

Merging #573 (9d5d0f8) into master (db5967d) will increase coverage by 0.00%.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##           master     #573   +/-   ##
=======================================
  Coverage   87.27%   87.27%           
=======================================
  Files         118      118           
  Lines        5711     5727   +16     
=======================================
+ Hits         4984     4998   +14     
- Misses        727      729    +2     

Impacted Files	Coverage Δ
etna/pipeline/base.py	91.89% <87.50%> (-3.35%)	⬇️

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