Fix native networks to work with generated future equals to horizon

[Mr-Geekman]

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?

Proposed Changes

1. Pass horizon instead of decoder_length in forecast.
2. Call make_future with decoder_length in pipeline
3. Fix tests for native networks.

Closing issues

Closes #925.

[github-actions]

🚀 Deployed on https://deploy-preview-936--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #936 (c754a50) into master (9aede87) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #936   +/-   ##
=======================================
  Coverage   84.99%   84.99%           
=======================================
  Files         133      133           
  Lines        7611     7611           
=======================================
  Hits         6469     6469           
  Misses       1142     1142           

Impacted Files	Coverage Δ
etna/models/base.py	87.59% <ø> (ø)
etna/pipeline/pipeline.py	97.29% <100.00%> (ø)

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[martins0n]

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