attacks: Make config validation skip fields with a default value

[tomtseng]

Changes

When running LoRA fine-tuning I got

Traceback (most recent call last):
  File "/workspace/TamperBench/scripts/user/tomtseng/mmlu_pro_chat_260211/run_mmlu_pro_variants.py", line 197, in <module>
    main()
  File "/workspace/TamperBench/scripts/user/tomtseng/mmlu_pro_chat_260211/run_mmlu_pro_variants.py", line 185, in main
    run_attack_and_evaluate(
  File "/workspace/TamperBench/scripts/user/tomtseng/mmlu_pro_chat_260211/run_mmlu_pro_variants.py", line 116, in run_attack_and_evaluate
    attacker = attack_cls(attack_config=attack_config_cls.from_dict(data=attack_config_data))
                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/TamperBench/src/tamperbench/whitebox/attacks/base.py", line 67, in from_dict
    cls._validate_dict_keys(data)
  File "/workspace/TamperBench/src/tamperbench/whitebox/attacks/base.py", line 105, in _validate_dict_keys
    raise ValueError(f"`{config_key}` is a mandatory field, and is not provided")
ValueError: `target_modules` is a mandatory field, and is not provided

which is a field introduced by #69. #69 tried to make this backwards compatible (it fills in the field later if it's not provided) but didn't actually test it. I think the fix here is to make the validator not flag the field.

Testing

Added unit tests, and they pass