Standardize naming for target outputs and attack signals across MIA attacks

Different MIA implementations currently use different names for similar values. Examples include:

• base: logits_target
• attack_p: attack_signal and audit_signal
• loss_trajectory: target
• LiRA and MS-LiRA: taget_model_logitsm, target_signals, shadow_models_signals, and sample_target_signals

Use the same naming pattern across all similar MIA attacks. The names should make it clear whether a variable contains the model’s original output or a signal calculated from that output. For example, use target_outputs and shadow_outputs or target_model_outputs and shadow_model_outputs for original model outputs.

It might be better with outputs than logits, because LeakPro also supports regression and forecasting models, which do not necessarily produce logits.