The hyperparams you can tune must be in the TrainingArguments you passed to your Trainer. If you have custom ones that are not in TrainingArguments, just subclass TrainingArguments and add them in your subclass.
The hp_space function indicates the hyperparameter search space (see the code of the default for optuna or Ray in training_utils.py and adapt it to your needs) and the compute_objective function should return the objective to minize/maximize.