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.