Add abstract predict method to pipelines

[Mr-Geekman]

Before submitting (must do checklist)

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

Proposed Changes

Look #716.

Closing issues

Closes #716.

[github-actions[bot]]

🚀 Deployed on https://deploy-preview-839--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #839 (55a0672) into master (01f47e5) will decrease coverage by 34.90%. The diff coverage is 22.22%.

@@             Coverage Diff             @@
##           master     #839       +/-   ##
===========================================
- Coverage   84.34%   49.44%   -34.91%     
===========================================
  Files         126      127        +1     
  Lines        7238     7303       +65     
===========================================
- Hits         6105     3611     -2494     
- Misses       1133     3692     +2559     

Impacted Files	Coverage Δ
etna/pipeline/base.py	80.65% <22.22%> (-13.88%)	:arrow_down:
etna/commands/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
etna/commands/backtest_command.py	0.00% <0.00%> (-97.06%)	:arrow_down:
etna/commands/forecast_command.py	0.00% <0.00%> (-94.88%)	:arrow_down:
etna/commands/__main__.py	0.00% <0.00%> (-87.50%)	:arrow_down:
etna/models/utils.py	14.28% <0.00%> (-85.72%)	:arrow_down:
etna/commands/resolvers.py	0.00% <0.00%> (-80.00%)	:arrow_down:
etna/analysis/outliers/density_outliers.py	22.44% <0.00%> (-75.52%)	:arrow_down:
etna/transforms/timestamp/time_flags.py	27.02% <0.00%> (-72.98%)	:arrow_down:
etna/transforms/timestamp/fourier.py	28.00% <0.00%> (-72.00%)	:arrow_down:
... and 79 more

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