Make optimizer initialization flexible

[jdb78]

Description

Optimizer can be initialized by passing string, class or function + fixing transformation for MLP

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 #602 (bc62f84) into master (007013e) will decrease coverage by 0.96%.
The diff coverage is 80.37%.

@@            Coverage Diff             @@
##           master     #602      +/-   ##
==========================================
- Coverage   90.54%   89.58%   -0.97%     
==========================================
  Files          20       24       +4     
  Lines        3312     3716     +404     
==========================================
+ Hits         2999     3329     +330     
- Misses        313      387      +74     

Flag	Coverage Δ
cpu	89.58% <80.37%> (-0.97%)	⬇️
pytest	89.58% <80.37%> (-0.97%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pytorch_forecasting/models/basic_rnn/__init__.py	0.00% <0.00%> (ø)
pytorch_forecasting/data/encoders.py	86.49% <78.12%> (+1.77%)	⬆️
pytorch_forecasting/models/nbeats/__init__.py	93.57% <83.33%> (-0.12%)	⬇️
pytorch_forecasting/models/base_model.py	89.92% <84.15%> (+1.59%)	⬆️
pytorch_forecasting/utils.py	81.94% <86.48%> (+1.38%)	⬆️
pytorch_forecasting/data/timeseries.py	93.35% <87.50%> (+0.32%)	⬆️
pytorch_forecasting/models/deepar/__init__.py	97.54% <88.23%> (-0.97%)	⬇️
pytorch_forecasting/metrics.py	93.45% <89.65%> (-0.72%)	⬇️
pytorch_forecasting/__init__.py	100.00% <100.00%> (ø)
pytorch_forecasting/data/__init__.py	100.00% <100.00%> (ø)
... and 14 more

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