Add QWEN tasks creation

• Replace Mistral-7B with QWEN endpoint
• Fix the prompt for open-ended answer evaluation
• Add a validator for single- and multi-choice questions such that for single-choice only one answer is correct, for multi-choice - at least one
• TasksCreator is now deprecated. The task generation should be done directly through the generate_task() method, and the answer evaluation through evaluate_open_ended().