outsource encode_documents() from PieDataModule.setup()

[ArneBinder]

This PR allows it to easily overwrite that functionality in downstream projects.

some context/motivation: ArneBinder/pie-datasets#200.

[codecov]

Codecov Report

❌ Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.53%. Comparing base (f99c96f) to head (5df85e1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pytorch_ie/datamodule.py	22.22%	7 Missing ⚠️

❌ Your patch status has failed because the patch coverage (22.22%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
- Coverage   78.34%   72.53%   -5.81%     
==========================================
  Files          32       32              
  Lines        2161     2163       +2     
  Branches      316      316              
==========================================
- Hits         1693     1569     -124     
- Misses        377      513     +136     
+ Partials       91       81      -10     

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