Optimize determine_num_steps function

[scanhex12]

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

Closing issues

closes #852

[github-actions]

🚀 Deployed on https://deploy-preview-864--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #864 (94ce38c) into master (61d9a80) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #864   +/-   ##
=======================================
  Coverage   84.75%   84.75%           
=======================================
  Files         132      132           
  Lines        7471     7473    +2     
=======================================
+ Hits         6332     6334    +2     
  Misses       1139     1139           

Impacted Files	Coverage Δ
etna/models/utils.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more