PointerNetworkTaskModuleForEnd2EndRE: add reversed relations

[ArneBinder]

This PR implements the parameters add_reversed_relations and symmetric_relations for PointerNetworkTaskModuleForEnd2EndRE (in analogy to their implementation in RETextClassificationWithIndicesTaskModule).

IMPORTANT: In production, this should make use of document.deduplicate_annotations() from pytorch-ie v0.31.3 ArneBinder/pytorch-ie#436 because otherwise duplicated annotations will occur in the predictions!

Potential follow-up: Outsource to ReversedRelationsMixture.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.49%. Comparing base (c481fb4) to head (2d6d1d0).

Files with missing lines	Patch %	Lines
...ules/taskmodules/pointer_network_for_end2end_re.py	92.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
- Coverage   95.52%   95.49%   -0.03%     
==========================================
  Files          61       61              
  Lines        5134     5173      +39     
==========================================
+ Hits         4904     4940      +36     
- Misses        230      233       +3     

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