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.

Locate the base, attack_p, loss_trajectory, LiRA, and MS-LiRA implementations and search for the listed target, shadow, logits, signal, and audit variable names. Standardize equivalent names so original model outputs are distinct from calculated signals, while supporting regression and forecasting outputs; verify that all affected attacks still work consistently.

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