[MNT] isolate pytorch-optimizer as soft dependency

[fkiraly]

Isolates pytorch-optimizer as soft dependency in a new soft dep set all_extras. #1616

The imports happen only in BaseModel, when resolving aliases for optimizer.

Isolation consists of two steps:

• replacing resolution of the alias with a request to install the package, and removing from resolution scope if not installed
• replacing the default optimizer "ranger" with "adam" if pytorch-optimizer is not installed (left at "ranger" if installed)

Deprecation messages and actions are added, to changne the default to "adam" from 1.2.0, in order to minimize the number of dependencies in default parameter settings

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Codecov Report

Attention: Patch coverage is 45.00000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 90.14%. Comparing base (95fa06c) to head (38daba9).

Files with missing lines	Patch %	Lines
pytorch_forecasting/models/base_model.py	45.00%	11 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1641      +/-   ##
==========================================
+ Coverage   90.10%   90.14%   +0.04%     
==========================================
  Files          32       32              
  Lines        4768     4780      +12     
==========================================
+ Hits         4296     4309      +13     
+ Misses        472      471       -1     

Flag	Coverage Δ
cpu	90.14% <45.00%> (+0.04%)	⬆️
pytest	90.14% <45.00%> (+0.04%)	⬆️

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