mlr3pipelines 0.8.0

• Added missing error for predicting with untrained PipeOps / Graphs.
• Fix: Corrected typo in the hyperparameter name use_parallel of PipeOpVtreat.
• Fix: Do not overwrite initial hyperparameter settings of bbotk::OptimizerBatchNLoptr in LearnerClassifAvg / LearnerRegrAvg's internal optimize_weights_learneravg function.
• Added new convenience function preproc() for easier training of or prediction with PipeOps or Graphs.
• Fix: PipeOpVtreat, PipeOpEncodeImpact, and PipeOpEncodeLmer now accept the more precise TaskSupervised instead of Task as input for training and prediction.
• Docs: Added missing documentation for the task_type of the input and output channels of PipeOps that inherit from PipeOpTaskPreproc and set a non-default task_type.
• Fix: PipeOpEncodeLmer, PipeOpADAS, PipeOpBLSmote, PipeOpSmote, and PipeOpSmoteNC no longer throw an error in case of empty target levels during training.
• Fix: PipeOpClassBalancing now handles unseen target levels by ignoring them during upsampling instead of producing NAs.