Add inverse_transform in TimeSeriesImputerTransform

[Mr-Geekman]

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

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?

Type of Change

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

• Add inverse_transform in TimeSeriesImputerTransform along with tests
• Little refactor of test_impute_transform.py
• Fix bug with inplace transform with option running_mean

Related Issue

#132.

Closing issues

Closes #132.

[codecov-commenter]

Codecov Report

Merging #135 (362c666) into master (d4b98a4) will decrease coverage by 1.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   88.06%   87.01%   -1.05%     
==========================================
  Files          66       66              
  Lines        2865     2873       +8     
==========================================
- Hits         2523     2500      -23     
- Misses        342      373      +31     

Impacted Files	Coverage Δ
etna/transforms/imputation.py	100.00% <100.00%> (ø)
etna/models/nn/deepar.py	73.91% <0.00%> (-26.09%)	⬇️
etna/models/nn/tft.py	75.00% <0.00%> (-25.00%)	⬇️
etna/transforms/pytorch_forecasting.py	92.20% <0.00%> (-7.80%)	⬇️
etna/loggers/base.py	92.30% <0.00%> (-1.93%)	⬇️

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[julia-shenshina]

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