Add method for determining the size of the context

[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?
• [ ] Did you update the CHANGELOG?

Proposed Changes

Look #844.

Closing issues

Closes #844.

[github-actions[bot]]

🚀 Deployed on https://deploy-preview-855--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #855 (06e878d) into inference (7ef6231) will increase coverage by 0.11%. The diff coverage is 93.82%.

@@              Coverage Diff              @@
##           inference     #855      +/-   ##
=============================================
+ Coverage      84.64%   84.75%   +0.11%     
=============================================
  Files            130      130              
  Lines           7450     7460      +10     
=============================================
+ Hits            6306     6323      +17     
+ Misses          1144     1137       -7     

Impacted Files	Coverage Δ
etna/models/base.py	87.54% <84.61%> (-0.19%)	:arrow_down:
etna/models/sarimax.py	94.81% <91.35%> (+1.16%)	:arrow_up:
etna/models/deadline_ma.py	91.56% <95.00%> (+4.26%)	:arrow_up:
etna/datasets/tsdataset.py	90.76% <100.00%> (+0.07%)	:arrow_up:
etna/models/autoarima.py	100.00% <100.00%> (+9.58%)	:arrow_up:
etna/models/catboost.py	100.00% <100.00%> (ø)
etna/models/holt_winters.py	98.86% <100.00%> (+0.04%)	:arrow_up:
etna/models/nn/deepar.py	100.00% <100.00%> (ø)
etna/models/nn/tft.py	100.00% <100.00%> (ø)
etna/models/prophet.py	98.85% <100.00%> (+0.04%)	:arrow_up:
... and 5 more

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