[MNT] isolate matplotlib as soft dependency

[fkiraly]

Isolates matplotlib as soft dependency in a new soft dep set all_extras. #1616

The imports happen in plot_sth methods throughout the code base, some attached to classes, some not.
This allows to use pytorch-forecasting without plotting or graphical logging, or use a different plotting backend manually.

Isolation strategy:

• where the purpose of the function is creating a plot or nothing else, absence of matplotlib raises an exception
• where matplotlib is additional or part of the logic, absence of matplotlib causes the specific parts to be skipped. Example: log_gradient_flow - this is crucial, as raising an exception would prevent the models from running.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 80.48780% with 8 lines in your changes missing coverage. Please review.

Project coverage is 90.10%. Comparing base (6061af6) to head (20f606d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pytorch_forecasting/data/timeseries.py	50.00%	2 Missing ⚠️
pytorch_forecasting/models/base_model.py	83.33%	2 Missing ⚠️
pytorch_forecasting/models/nbeats/__init__.py	83.33%	1 Missing ⚠️
pytorch_forecasting/models/nhits/__init__.py	83.33%	1 Missing ⚠️
...ing/models/temporal_fusion_transformer/__init__.py	87.50%	1 Missing ⚠️
pytorch_forecasting/utils/_dependencies.py	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1636      +/-   ##
==========================================
- Coverage   90.19%   90.10%   -0.10%     
==========================================
  Files          32       32              
  Lines        4734     4768      +34     
==========================================
+ Hits         4270     4296      +26     
- Misses        464      472       +8     

Flag	Coverage Δ
cpu	90.10% <80.48%> (-0.10%)	⬇️
pytest	90.10% <80.48%> (-0.10%)	⬇️

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