Extend iterative process builder and the LR scheduling iterative process to...

Extend iterative process builder and the LR scheduling iterative process to accept an optional"datasset preprocessing computation.

This allows for pushing the dataset preprocessing methods down to the client
executors, which is required for multimachine simulations since stateful
datasets (e.g. datasets which suffling) cannot be serialized.

PiperOrigin-RevId: 321835968