Wandb Logger does not work unless pytorch is installed

[martins0n]

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

PLwandb logger initialization in getter only when it's needed

Related Issue

Closing issues

closes #335

[martins0n]

https://colab.research.google.com/drive/1heiAllLriCf5JKjr4R9gbSmAWg5SEaWG?usp=sharing

[codecov-commenter]

Codecov Report

Merging #340 (05fb5b5) into master (af51f3e) will decrease coverage by 0.00%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
- Coverage   87.67%   87.67%   -0.01%     
==========================================
  Files          98       98              
  Lines        4820     4818       -2     
==========================================
- Hits         4226     4224       -2     
  Misses        594      594              

Impacted Files	Coverage Δ
etna/loggers/wandb_logger.py	32.18% <0.00%> (-1.53%)	⬇️
etna/loggers/__init__.py	100.00% <100.00%> (ø)

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[iKintosh]

This way you can revoke approve