add fit_called checks in global models

[hrzn]

Fixes #764

[codecov-commenter]

Codecov Report

Merging #944 (f6711f5) into master (611e411) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #944   +/-   ##
=======================================
  Coverage   91.70%   91.70%           
=======================================
  Files          74       74           
  Lines        7398     7399    +1     
=======================================
+ Hits         6784     6785    +1     
  Misses        614      614           

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	96.77% <100.00%> (-0.02%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.35% <100.00%> (+0.04%)	⬆️

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