[Bug-fix] Refactor warning about global normalization to forecaster.py

[judussoari]

🔬 Background

Fixes #1053
This code portion produced a warning on every single training step, which leads to huge, unnecessary log output.

if not self.config_normalization.global_normalization:
     log.warning("When Global modeling with local normalization, metrics are displayed in normalized scale.")
else:
     ...

🔮 Key changes

Moved the warning to m.fit()

📋 Review Checklist

• I have performed a self-review of my own code.
• I have commented my code, added docstrings and data types to function definitions.
• I have added pytests to check whether my feature / fix works.

[codecov-commenter]

Codecov Report

Merging #1109 (bbdb8bf) into main (ebbe06a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1109   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files          21       21           
  Lines        4799     4800    +1     
=======================================
+ Hits         4326     4327    +1     
  Misses        473      473           

Impacted Files	Coverage Δ
neuralprophet/forecaster.py	87.89% <100.00%> (+0.02%)	⬆️
neuralprophet/time_net.py	90.78% <100.00%> (-0.02%)	⬇️

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[ourownstory]

Great, thank you!