generate adversarial samples API needed

[Opdoop]

After updating, attack_evals.DefaultAttackEval no longer exist. The test below is broken. https://github.com/thunlp/OpenAttack/blob/bfedfa74f37c69db6d7092d9cc61822ee324919d/slow_tests/test_multi_process.py#L32 Because we don't have DefaultAttackEval anymore, it seems like we don't have a function to generate adversarial examples. The TODO below implementation is now emergent. https://github.com/thunlp/OpenAttack/blob/bfedfa74f37c69db6d7092d9cc61822ee324919d/OpenAttack/attack_eval/attack_eval.py#L257