Make `TimeSeriesImputerTransform` vectorized

[Mr-Geekman]

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

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?

Type of Change

• [ ] Examples / docs / tutorials / contributors update
• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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

Make TimeSeriesImputerTransform vectorized

Related Issue

#748.

[github-actions[bot]]

🚀 Deployed on https://deploy-preview-760--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #760 (5b8c41d) into master (4b034be) will decrease coverage by 32.78%. The diff coverage is 10.34%.

@@             Coverage Diff             @@
##           master     #760       +/-   ##
===========================================
- Coverage   83.50%   50.71%   -32.79%     
===========================================
  Files         122      122               
  Lines        6734     6743        +9     
===========================================
- Hits         5623     3420     -2203     
- Misses       1111     3323     +2212     

Impacted Files	Coverage Δ
etna/transforms/missing_values/imputation.py	24.69% <10.34%> (-73.92%)	:arrow_down:
etna/commands/__init__.py	0.00% <0.00%> (-100.00%)	:arrow_down:
etna/commands/backtest_command.py	0.00% <0.00%> (-96.43%)	:arrow_down:
etna/commands/forecast_command.py	0.00% <0.00%> (-93.94%)	:arrow_down:
etna/commands/__main__.py	0.00% <0.00%> (-87.50%)	: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/datasets/datasets_generation.py	27.02% <0.00%> (-72.98%)	: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 71 more

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