Fix continuation tokens after distributed gathering

[Excelius-Wang]

Summary

• slice gathered continuation tensors across both the choice and token dimensions
• strip the -1 padding used only to make continuations gatherable
• extend the existing padding test to verify every choice is preserved and token counts exclude padding

Fixes #1170.

Testing

• pytest tests/unit/models/test_transformers_model.py::TestTransformersModelProcessing -q
• pytest tests/unit/metrics/test_normalizations.py -q
• ruff check src/lighteval/models/transformers/transformers_model.py tests/unit/models/test_transformers_model.py
• ruff format --check src/lighteval/models/transformers/transformers_model.py tests/unit/models/test_transformers_model.py