Allow multi-target prediction with Baseline model

[jdb78]

Checklist

• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Make sure to have fun coding!

[codecov-commenter]

Codecov Report

Merging #603 (9468ece) into master (bc62f84) will increase coverage by 0.29%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #603      +/-   ##
==========================================
+ Coverage   89.58%   89.87%   +0.29%     
==========================================
  Files          24       24              
  Lines        3716     3714       -2     
==========================================
+ Hits         3329     3338       +9     
+ Misses        387      376      -11     

Flag	Coverage Δ
cpu	89.87% <93.33%> (+0.29%)	⬆️
pytest	89.87% <93.33%> (+0.29%)	⬆️

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Impacted Files	Coverage Δ
pytorch_forecasting/models/base_model.py	89.92% <ø> (ø)
pytorch_forecasting/models/baseline.py	94.73% <93.33%> (+51.87%)	⬆️

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