Preprocesses predictor data (x) into a format suitable for Keras models
built with the Functional API. Handles both tabular data and list-columns
of arrays (e.g., for images), supporting multiple inputs.
process_x_functional(x)A list containing:
x_proc: The processed predictor data (matrix or array, or list of arrays).
input_shape: The determined input shape(s) for the Keras model.
A data frame or matrix of predictors.