Fix argparse dataset restriction to allow test datasets

[TosakaUCW]

Problem

src/generate_model_answers.py previously restricted --dataset via argparse choices=LIST_OF_DATASETS. This caused runs to fail at argument parsing time when using dataset names not present in the allowlist (e.g., triviaqa_test).

Change

• Removed the choices restriction for --dataset
• Made --dataset required and added a clearer help message

Impact

Allows running the script with both training and test dataset names (including *_test variants) without being blocked by argparse.

How to test

python src/generate_model_answers.py --model <model_name> --dataset triviaqa_test